The Process: First, D-Smash an opponent. After a short period, they will leave the stun animation and fall backwards, if their recovery isn't good enough, they will touch the ground before being able to jump again. When they are landing, their stun cooldown is being refreshed, therefore allowing ZSS to D-Smash them again, succeeding in stunning them once more. If you have good timing, they will not be able to even shield the attacks. Thus, on certain characters, ZSS has a 100% guaranteed combo.
The Escape: Now, DI can help quite a bit, lowering the "guaranteed" percent a healthy amount. If the opponent DIs their joystick vertically UP (horizontal doesn't matter) at the moment they leave the stun, they will have more time in the air, which will give them more time to recover, and more time to jump. Sadly, there is never any "bad" DI, there is either useless/no DI or good DI. Now, like I said before, charging the D-Smash will knock an opponent higher into the air, giving them more time to jump out of the chain, so try to use the shortest charge possible, or just c-stick it altogether.
The Solution: On the other hand, degenerating your D-Smash by using it repeatedly will make it more difficult for your opponent to escape. This has great implications! The best way to keep your opponent in a D-Smash chain longer is to D-Smash chain them longer! What this means is that if you start a D-Smash chain at a low percent, then by the time you get up to the character's limit, you'll have degenerated your D-Smash enough that you will be able to continue the chain!
The List: Alright, I bet your mouths are watering with curiosity, wondering who the poor victims are. Well with no further ado, here are my findings:
(None and Full refer to the amount of degeneration, so None means you haven't used the D-Smash for 9 moves, and Full means you've used it 8 times in a row.)
--------------------- with DI ------- w/out DI ----------
Character : ---- None - Full ------ None - Full --------
Bowser : --------- 0% - 0% -------- 2% - 24% -------
Diddy Kong : ---- 0% - 0% -------- 0% - 13% -------
Link : ------------- 0% - 0% -------- 1% - 24% -------
Sheik : ---------- 22% - 55% ------ 39% - 91% ------
Ganondorf : ----- 0% - 4% -------- 27% - 63% ------
Zero Suit Samus: 0% - 0% -------- 0% - 3% --------
Metaknight : ----- 0% - 0% -------- 0% - 14% ------
King Dedede : --- 0% - 0% --------- 0% - 4% -------
Fox : ------------ 58% - 120% ----- 58% - 121% ----
Falco : ---------- 20% - 44% ------- 35% - 87% -----
Wolf : ----------- 39% - 91% ------- 56% - 117% ---
Captain Falcon : 12% - 44% ------- 39% - 93% -----
Squirtle : -------- 0% - 0% --------- 0% - 15% ------
Ike : ------------- 0% - 0% --------- 0% - 4% --------
Lucas : ---------- 0% - 0% --------- 0% - 1% --------
Sonic : ---------- 0% - 0% --------- 0% - 2% --------
Basic Explanation: If you connect with a D-Smash while the listed opponent is below the listed percent, before the D-Smash, you will always be able to connect with another D-Smash. For instance, if you were D-Smash a Sheik at 21%, you will always be able to D-Smash her again. Even though her percent will go up to 32% from the D-Smash, since she was below the listed percent before the D-Smash, then you are guaranteed another D-Smash.