Tool Assisted Speed-runs are those done with the assistance of programs that allow the player to re-record segments, edit them, time actions on a frame by frame basis to manipulate luck, and all sorts of other funky things that wouldn’t strictly be possible when playing normally. The aim of these is to get to the end of the game, from a clean state, as quickly as possible, though sometimes speed is sacrificed for entertainment value.
While very often this is seen done for platformers, it isn’t limited to that genre alone. In this case, SD3 has finally been TASed, and completed in just under 3 and a half hours by praetarius3. You can watch it on YouTube here, or download from the tasvideos.org page here. Also, if you want to understand some of the things going on a bit better, check out the author’s comments here.
When watching the video, turn on closed captions in YouTube to get some background as to what is going on, as this plays on a non-patched version of the game.