They are made by going through any game Frame By Frame and with savestates to allow Inhumanly possible reflexes, Trigger glitches, and allow the game to be run with absolute precision, then played back at full speed.
Some prove to be quite impressive.
Watch Youtube Video:
http://www.youtube.com/watch?v=4C1XaPIWQmM
If you could do the same thing to Real Life, What would you do?