This is my theory on how hitstun deterioration works. I do not come prepared with videos to support my case, but with past experiences and extensive labbing. Bear in mind that this is all speculation, but that it is speculation with some in-game evidence to back it up.
First of all, HSD is a timer. From the very first hit of the combo, it is activated and begins ticking downward. Every move has a base hitstun value and a minimum hitstun value. As the timer ticks downward, the moves slowly transition from their base value to their minimum value. The minimum value for each move differs from move to move, but I believe the lowest value of any move is 4 frames. I could be wrong, though. That is the system in a nutshell; a timer that decreases a move’s hitstun as it nears zero. It seems really simple, and that’s because I haven’t started discussing the different factors that affect the timer.
As you may know, all the timers in the game, be it for X Factor or powerup hypers, freeze momentarily when the person using the timer lands a hit. I call this the Timer Freeze Property. This property also applies to the HSD timer, from what I can tell. Although it’s hard to create a test that tells for sure, it makes logical sense and it seems to pop up a bit in the game when people delay their moves too much.
The next interesting property is that, from what I can tell, a certain amount of time is re-added to the HSD timer when the opponent is launched, and a flying screen occurs. This theory has great merit to it; one who regularly plays Doctor Strange can relate to this property quite well. You see, in Strange’s Air Impact Palm loops, there comes a point where if you try to do Jumping MMH xx Impact Palm just one more time, they fall out. If you launch them, however, the Jumping MMH xx Impact Palm works again.
Now, before HSD was fully explored, the common argument against this theory was that the flying screen HSD and regular HSD were separate. However, with some tricky timing, Strange can do an entire combo without launching, and when he finally does launch, his air series will not work. This can be verified with many other characters, too, making the theory presumably valid.
Something that I haven’t quite figured out the details of are the way special opponent states affect the HSD timer. From what I have observed, however, I can guess that the HSD timer never stops running, and that not even special states alter the flow of the timer. In several combos with groundbounce starters, I’ve found that the timer definitely continues running during bouncing states. It also definitely continues running during hard knockdowns, and soft knockdowns mark the end of a combo. That leaves two special opponent states that I haven’t figured out.
•Special hitstun state
A special hitstun state is a state where the opponent is placed in some sort of fixed or mechanically different hitstun, such as a spinning knockdown or the fixed hitstun caused by Strange’s Air Impact Palm. It is unknown whether or not the timer runs during these states, but I firmly believe that it does. Sadly, I have no evidence to support this except for experience with combos. As for crumple states, I believe it runs there as well, although it may run slower than normal.
Finally, TAC’s affect hitstun in some way that I cannot exactly tell. I know the amount of hitstun the incoming character has to work with is proportionate to the length of the original character’s combo, but the extent of this is unknown. Upon a TAC, the HSD timer is temporarily reset to a given amount, where it appears to stay until the new character lands. Once the new character touches the ground, the HSD timer is set to the original time, minus the time of the entire combo. That’s what I can tell. In case you weren’t aware, a TAC also puts the opponent into a soft knockdown. I’m sure you knew that, though.
Also, this goes without saying. The following moves are never affected by HSD, despite the fact that the timer keeps running.
•Sequences in which moves hit more frequently than once every four (?) frames
That pretty much wraps up my knowledge of HSD.