I'm still holding out hope that this glitch will have some time-saving application, so I've spent some time trying to better understand it
Demonstration .fm2
Summary of what I think I know: Normally $75 is always 0. It's read from and written to once every frame of player control, and appears to be used simply to set the X register so a DEX will set the N flag (though if that's the intended behavior, why mess with $75 at all when a LDX #$00 could have done the same job?). But if Ryu takes damage at the same time as delivering the killing blow on a boss, then $75 gets set to 4. So, when that DEX mentioned earlier occurs and that N flag isn't set, the game branches into a loop to decrement the X register to the expected 0—a loop that would
normally come much later in the frame. In this loop the value of $74 (which appears to be used as an offset for the different object positions in memory, 0-7) is also being decremented, which ends up meaning that speed values are sometimes not added to enemy
projectile positions. It's a small difference, but as the linked movie shows, though with an undesirable result, it can be enough to matter.
What I'm missing is why this only occasionally happens. The number of objects on screen and the affected projectile's position (0-7) in the objects active at the time are probably factors, but I haven't picked up on a pattern there yet.
The other aspect I haven't really examined is the initial triggering of the glitch, when Ryu takes a hit on boss death (I'm pretty sure it can happen when other stuff takes damage instead of Ryu too), but here are trace logs feos posted when we were discussing this on IRC a while back:
no damage,
damage
trace logs comparing normal behavior with an occurrence of a bullet being affected (the 'snips' are for long , identical segments and I was using the begin/end to view the logs in np++ with Ruby syntax so differently branching segments could be collapsed):
$75=0,
$75=4
EDIT: Couple of things I overlooked. Defeating the Masked Devil/mind-control orb with the sword or jump-n-slash always results in $75=02 and after fast-forwarding through BoltR's and Slotermeyer's runs I noticed that they both finish the level 3 boss with the firewheel which gets $75=04 too, and it turns out that killing any boss with the throwing star, windmill star or firewheel has the same effect—or $75=05 instead of 02 in the case of the first of the final three bosses.