or he was timing a multi-tap delay unit.

before moving to the 2nd laptop for efx, that pedal was used to control a Lexicon (or DP4, sometimes) efx box, which was routed out the Send and then back into an empty channel strip on the mixer, making the efx another ~instrument. he could mix all the way out of both decks, 'play' the efx track, and come back in with a record (or sampler) whenever he felt like it.

what you're suggesting is not possible with fs. pitch is still controlled with the decks, manually.

it's theoretically possible he's tapping into the 2nd computer which is now a Live/multi-effects unit but in that case he would have been interacting with an AIFF file and/or the new efx send in Ableton, not a FS mp3 file you're hearing on the decks.

there's also a lot of tapping that goes on with the c-loops sampler, but that's all by hand.

he also just likes to tap his foot a lot.