Apparently, if you do a neutral spin jump (press forward+jump, then release forward within 5/6 frames) and hold up on the dpad, then you will fall at 1/4 of the normal rate. Pressing left or right (or letting go of up) breaks you out of this little hover.
I’m struggling to think of any application for this trick, aside from dealing with respawning blocks in some niche situations.
(This discovery elicited a “w h a t” from a current AM2R dev, and a “oh I already knew that” from someone I don’t know.)
Somewhat related to this, there is some vestigial code in several Samus movement functions that is supposed to allow her to jump while in acid. However, the flag the functions in question check (acidContactFlag) is never set because (a) it’s cleared every frame and (b) those functions perform Samus-BG collision detection after checking the acid flag. I attempted to hack this functionality back in, but the result feels really janky (mostly because you have to be in a falling state to initiate these jumps).