he's passing through enemies altogether. So what are those speedups doing that makes this possible?

Nemo2342, your question is answered here: http://tasvideos.org/CommonTricks.html#fast_motion_collision_abuse_ In short, "motion" in a motion picture is about being somewhere in one picture and somewhere else in the other picture. When moving fast enough, there exists no picture where you are inside the enemy -- you just teleport through it. Because of reasons of limited processing power (or sometimes, programmers' laziness), the game only checks each instant that are being displayed on screen, and does not extrapolate paths and movement to find out what supposedly happens between two frames. This is called quantization of time, or iteration, if I recall correctly.