The "Simulator" aspect refers to the game's focus on replayability. Unlike the standard Undertale fight, which is a one-off story event, these simulators are designed for loop-based combat. You fight wave after wave of "Gaster Blasters," bones, and telekinetic grabs until your HP hits zero.

A major feature unique to the simulator genre is the "Re-record" mechanic. Instead of a standard "Retry" button, you hit "REWIND." The screen reverses, bones fly backwards, and you restart the phase. However, each re-record leaves a "ghost" of your previous attempt on the screen, cluttering the battlefield with translucent after-images of your past failures.

Sans’ eye glitches red → Player can attack. If hit: “tracking resets. glitch gauge -50%.” If miss: “headache hum activates. buttons scrambled.”

: Similar to the original Sans fight, success relies heavily on memorizing complex, rhythmic patterns.