I looked at it in FCEU's debugger. The code at $91D9 is "DEC $075A", which decreases the byte by one. Same result as SBC #1, but a simpler method. The game genie code changes that to "LDA $075A", $075A is unchanged in that case.

About the infinite JMP loop, you guessed it. It just sits there during the idle time, while all the code is ran from the NMI routine.