Can someone explain how he manages to play pong, snake, and display pi? It's absolutely mind-boggling.

Long story short - The TAS sends so many inputs per frame that it essentially overwrites the code with its own code in one form or another. You'd actually be better of asking over on TASVideos.org rather than the game mods.

There's a page over there explaining how to do it and may give you a little insight as to how they go about it. http://tasvideos.org/ArbitraryCodeExecutionHowTo.html

The author provides a great description of it in his submission of it to tas videos http://tasvideos.org/3894S.html, but without an understanding of the gritty details of ACE and CPU/RAM hardware functionality, it won't really make much sense to you (and it's not super easy to explain either).