freedoom is he using a mouse? is that the tool he's using for assistance?
fluffy TASes work by replaying a controller input to the emulator with frame-level precision. Everything done in a TAS could theoretically be done by a hacked controller on a real system. They don't involve any modification to the game logic, just exploits to existing game logic that make assumptions based on how controllers work (and which use input timing for random number genration and so on).
Cyberblah I've always wondered about these tool assisted speed runs. They're trying to push the game to its limits, but do they actually end up exploiting tiny flaws in the emulation? Can you even tell?
fluffy Emulation is extremely accurate nowadays. Also, these speedruns are often verified on multiple emulators, and the gold standard one appears to be FCEUX which is intended to be as close as you can get to actual hardware, while providing as many "ins" to inspecting the low-level state of the hardware as possible.