pssh, dude used the bar
It's pretty much just memorization at that point. Do the song over and over at slower speeds and ramp it up till you have it memorized at normal speed or faster. He could probably do it with his eyes closed. But yea, the only way you are going to move your feet that fast is to use the bar.
Yasu is like the current king of DDR, guy can triple A most anything.
Here's a link to when he AAA'd max 300 (pretty much the hardest standard liscensed DDR song. There are others that people have made for stepmania and stuff that are harder, but they aren't official songs) and the pic got up on Fark for the chop:
http://aij.ddruk.com/ddryasufark.html