Some stages, such as Summit, behave very badly when reversed. Castle Siege can crash when reversed as it tries to load a new area backward (I bet Lylat Cruise has the same issue). Use reversing with caution.
The numbers like BF800000 are 8-bit hexadecimal representations of floating point numbers. You don't have to understand what that is; you can use the following website to convert any number you like:
http://babbage.cs.qc.edu/IEEE-754/Decimal.html
BF800000 is -1, 00000000 is 0 (alternatively, 80000000 is "-0"), 3F000000 is 1/2, and 3F800000 is 1. There is no reason you have to be restricted to slowing stages down or exactly reversing them. You could use 40000000 to make a stage animate at double speed, or you could go really crazy and use C0000000 to make a stage animate at double speed in reverse. Using numbers that are way too big, especially 7FFFFFFF and FFFFFFFF which are infinities, is likely to cause crashing so don't do it.
Here is a stage ID list which you will need.
Code:
Stage ID list
00 (0) - Battle [nothing]
01 (1) - BattleField (Battlefield)
02 (2) - Final (Final Destination)
03 (3) - Dolpic (Delfino Plaza)
04 (4) - Mansion (Luigi's Mansion)
05 (5) - MarioPast (Mushroomy Kingdom)
06 (6) - Kart (Mario Circuit)
07 (7) - Donkey (75m)
08 (8) - Jungle (Rumble Falls)
09 (9) - Pirates (Pirate Ship)
0A (10) - Zelda2 (Bridge of Eldin, unused by game)
0B (11) - Norfair (Norfair)
0C (12) - Orpheon (Frigate Orpheon)
0D (13) - Crayon (Yoshi's Island [Brawl])
0E (14) - Halberd (Halberd)
0F (15) - TestHalberd_00 [nothing]
10 (16) - TestHalberd_01 [nothing]
11 (17) - TestHalberd_02 [nothing]
12 (18) - Karby2 [nothing]
13 (19) - Starfox (Lylat Cruise)
14 (20) - Stadium (Pokémon Stadium 2)
15 (21) - Tengan (Spear Pillar)
16 (22) - Fzero (Port Town Aero Dive)
17 (23) - Ice (Summit)
18 (24) - Gw (Flat Zone 2)
19 (25) - Emblem (Castle Siege)
1A (26) - TestEmblem_00 [nothing]
1B (27) - TestEmblem_01 [nothing]
1C (28) - Madein (WarioWare, Inc.)
1D (29) - Earth (Distant Planet)
1E (30) - Palutena (Skyworld)
1F (31) - Famicom (Mario Bros.)
20 (32) - Newpork (New Pork City)
21 (33) - Village (Smashville)
22 (34) - Metalgear (Shadow Moses Island)
23 (35) - Greenhill (Green Hill Zone)
24 (36) - Pictchat (PictoChat)
25 (37) - Plankton (Hanenbow)
26 (38) - ConfigTest (control configuration area, very bad camera)
27 (39) - Viewer [nothing]
28 (40) - Result (Results screen, flat and black)
29 (41) - DxShrine (Temple)
2A (42) - DxYorster (Yoshi's Island [Melee])
2B (43) - DxGarden (Jungle Japes)
2C (44) - DxOnett (Onett)
2D (45) - DxGreens (Green Greens)
2E (46) - DxPStadium (Pokémon Stadium)
2F (47) - DxRCruise (Rainbow Cruise)
30 (48) - DxCorneria (Corneria)
31 (49) - DxBigBlue (Big Blue)
32 (50) - DxZebes (Brinstar)
33 (51) - Oldin (Bridge of Eldin)
34 (52) - Homerun (Homerun Contest)
35 (53) - Edit (Stages made using the Stage Builder, specifically the last one you loaded or a crash if none at all)
36 (54) - Heal (All-Star Resting Stage)
37 (55) - OnlineTraining (WiFi Waiting Room)
38 (56) - TargetBreak (Break The Targets)
39 (57) - CharaRoll [nothing]
3A (58) - General [nothing]
3B (59) - Adventure [nothing]
3C (60) - Adventure0 [nothing]
3D (61) - Adventure2 [nothing]
3E (62) - AdvMeleeTest [nothing]
3F (63) - AdvMelee [nothing]
40 (64) - BattleS [nothing]
41 (65) - BattleFieldS [nothing]
42 (66) - Max [nothing]
43 (67) - Unknown [nothing]
I'm pretty sure all the stages I labeled as [nothing] just crash the game if you force them to load since there are no corresponding files to load and they're probably either artifacts of development or things that are used for internal processes that aren't actual battles (like I bet CharaRoll is called when the credits are rolling). I think both versions of Bridge of Eldin point to the same file, but the second one is definitely the one the game uses so it's the one you should use.
You should also be warned that stages which are frozen during their intro animations will not be while using this code. There is another version which is supposed to respect that initial freezing, but it causes other odd behavior on those stages. The only stages, to my knowledge, that are normally frozen during their introductions are the Mushroomy Kingdom, Rumble Falls, and Halberd. This only causes a problem on Rumble Falls which, when not frozen during its introduction, sometimes causes the Pokemon Trainer's Pokemon to spawn in the air which results in them being stuck in t-pose.
If I recall, this code has some way to distinguish between sub-stages (Mushroomy Kingdom 1-1 versus 1-2, different target tests, different forms of Spear Pillar), but I don't remember how.
