I have been exploring just which characters can force jump breaks on which others, and I've formulated several hypotheses on the matter. Some of this contradicts previous research I had done; I apologize for misleading anyone and will say that anything I posted previously that is contradicted here is wrong. I noticed that the same set of characters seemed to be susceptible to being forced to jump break consistently. I don't think characters have individual heights that are considered when they are grabbed; I believe all characters are considered either "tall" or "short" for the calculation of jump break situations.
Tall characters:
Peach, Bowser, Donkey Kong, Diddy Kong, Link, Zelda, Sheik, Ganondorf, Samus, Zero Suit Samus, Pit, R.O.B., King Dedede, Fox, Falco, Wolf, Captain Falcon, Charizard, Lucario, Marth, Ike, Snake, Sonic
Short characters:
Mario, Luigi, Yoshi, Wario, Toon Link, Ice Climbers, Kirby, Meta Knight, Olimar, Pikachu, Squirtle, Ivysaur, Jigglypuff, Ness, Lucas, Mr. Game & Watch
On flat ground, no characters can force the tall characters to jump break except for Snake and Yoshi. A wide variety of characters can force the short characters to jump break. In terms of "grab heights", the characters similarly are, for the most part, in one of two groups. Some characters grab "high", and others grab "low". As you might surmise, the "high grabbers" are the ones who can force the short characters to jump break. This distinction only matters on flat ground for the most part.
High grabbers:
Peach, Bowser, Donkey Kong, Wario, Zelda, Sheik, Ganondorf, Zero Suit Samus, Captain Falcon, Ivysaur, Marth, Ike
Low grabbers:
Mario, Luigi, Diddy Kong, Link, Toon Link, Samus, Pit, Ice Climbers, R.O.B., Kirby, Meta Knight, King Dedede, Fox, Falco, Wolf, Pikachu, Squirtle, Charizard, Lucario, Jigglypuff, Ness, Lucas, Mr. Game & Watch, Sonic
Special characters:
Yoshi, Olimar, Snake
The special characters will be addressed in a moment.
Slopes are very simple. I had strange, inconsistent results with very short slopes (such as on Castle Siege's first form: this slope is really weird) or slopes that keep changing (such as Lylat Cruise in general), but that's probably just experimental error. On sufficiently long slopes that are not VERY slight (Yoshi's Island Brawl's middle section of the main platform doesn't count and seems to be treated like flat ground), the rule is simple. If you are not one of the three special characters, you always hold the foe on the ground when facing upward on a slope and always hold the foe in the air when facing downward on a slope. This leads to some hilariously awkward forced jumpbreaks; Squirtle forcing Bowser to jump break on Distant Planet just looks wrong. Of course, if you hold an opponent over an edge, you also can force a jump break.
Snake and Olimar are very similar but opposite. Snake ALWAYS holds opponents up in the air no matter what, but opponents can still ground break if he is pummeling them. Olimar holds opponents on the ground with the only exception of the case where he's holding an opponent over an edge (that is, no matter how steep the slope, Olimar will still hold them on the ground). Yoshi has famously special mechanics that make jump breaking the only option for the opponent even if he is pummeling except in the obscure case where the opponent breaks out during the first pummel which is done at the first possible moment which results in a ground break (thanks to mmac on the Yoshi research).
What this means, in short, is that figuring out when you can force jump breaks or are going to be holding characters on the ground is very simple. You don't have to think about how tall a character looks or guess how much a slope matters. Victims fall into exactly two groups (that you can pretty easily memorize), and grabbers fall into two groups as well except have three special cases.
For those who wish to use this knowledge to look into exploiting ground breaks as opposed to jump breaks, this old thread of mine might have some use to you:
http://www.smashboards.com/showthread.php?p=5393749
Also, I discovered a funny glitch while testing this. On Castle Siege's first form, grab Sonic with Squirtle while on the slope facing downhill. Sonic very quickly flashes between two poses; it's really funny. Sonic seems to ground break by the way; like I said earlier, this slope is really weird and doesn't seem to follow the rules that other slopes do (even that ice slope on the Summit that seems even less steep follows the rules...).
Do note that this is all mostly a hypothesis. It would take thousands of trials to be able to conclude definitively that what I'm saying is true. I'm reasonably sure, but I do not make any guarantees. Anyway, I hope this was useful, and have fun abusing Wario on those slopes!
Tall characters:
Peach, Bowser, Donkey Kong, Diddy Kong, Link, Zelda, Sheik, Ganondorf, Samus, Zero Suit Samus, Pit, R.O.B., King Dedede, Fox, Falco, Wolf, Captain Falcon, Charizard, Lucario, Marth, Ike, Snake, Sonic
Short characters:
Mario, Luigi, Yoshi, Wario, Toon Link, Ice Climbers, Kirby, Meta Knight, Olimar, Pikachu, Squirtle, Ivysaur, Jigglypuff, Ness, Lucas, Mr. Game & Watch
On flat ground, no characters can force the tall characters to jump break except for Snake and Yoshi. A wide variety of characters can force the short characters to jump break. In terms of "grab heights", the characters similarly are, for the most part, in one of two groups. Some characters grab "high", and others grab "low". As you might surmise, the "high grabbers" are the ones who can force the short characters to jump break. This distinction only matters on flat ground for the most part.
High grabbers:
Peach, Bowser, Donkey Kong, Wario, Zelda, Sheik, Ganondorf, Zero Suit Samus, Captain Falcon, Ivysaur, Marth, Ike
Low grabbers:
Mario, Luigi, Diddy Kong, Link, Toon Link, Samus, Pit, Ice Climbers, R.O.B., Kirby, Meta Knight, King Dedede, Fox, Falco, Wolf, Pikachu, Squirtle, Charizard, Lucario, Jigglypuff, Ness, Lucas, Mr. Game & Watch, Sonic
Special characters:
Yoshi, Olimar, Snake
The special characters will be addressed in a moment.
Slopes are very simple. I had strange, inconsistent results with very short slopes (such as on Castle Siege's first form: this slope is really weird) or slopes that keep changing (such as Lylat Cruise in general), but that's probably just experimental error. On sufficiently long slopes that are not VERY slight (Yoshi's Island Brawl's middle section of the main platform doesn't count and seems to be treated like flat ground), the rule is simple. If you are not one of the three special characters, you always hold the foe on the ground when facing upward on a slope and always hold the foe in the air when facing downward on a slope. This leads to some hilariously awkward forced jumpbreaks; Squirtle forcing Bowser to jump break on Distant Planet just looks wrong. Of course, if you hold an opponent over an edge, you also can force a jump break.
Snake and Olimar are very similar but opposite. Snake ALWAYS holds opponents up in the air no matter what, but opponents can still ground break if he is pummeling them. Olimar holds opponents on the ground with the only exception of the case where he's holding an opponent over an edge (that is, no matter how steep the slope, Olimar will still hold them on the ground). Yoshi has famously special mechanics that make jump breaking the only option for the opponent even if he is pummeling except in the obscure case where the opponent breaks out during the first pummel which is done at the first possible moment which results in a ground break (thanks to mmac on the Yoshi research).
What this means, in short, is that figuring out when you can force jump breaks or are going to be holding characters on the ground is very simple. You don't have to think about how tall a character looks or guess how much a slope matters. Victims fall into exactly two groups (that you can pretty easily memorize), and grabbers fall into two groups as well except have three special cases.
For those who wish to use this knowledge to look into exploiting ground breaks as opposed to jump breaks, this old thread of mine might have some use to you:
http://www.smashboards.com/showthread.php?p=5393749
Also, I discovered a funny glitch while testing this. On Castle Siege's first form, grab Sonic with Squirtle while on the slope facing downhill. Sonic very quickly flashes between two poses; it's really funny. Sonic seems to ground break by the way; like I said earlier, this slope is really weird and doesn't seem to follow the rules that other slopes do (even that ice slope on the Summit that seems even less steep follows the rules...).
Do note that this is all mostly a hypothesis. It would take thousands of trials to be able to conclude definitively that what I'm saying is true. I'm reasonably sure, but I do not make any guarantees. Anyway, I hope this was useful, and have fun abusing Wario on those slopes!
