[Buji]

As I am not in Underworld I can't say what I am saying is a 100% correct on this but here we go:-

The track/instruments will have already been sequenced in Logic Audio or a similar programme, so when you load the track up in Logic it will synch with the desk. As for mixing into another track as far as I know Underworld have 2 Main Mac's with Logic on with one sending Midi clock to all the other comps/laptops they have keeping in synch with the tempo, so when one track finishes on logic on one computer, they can set the other one going on the other computer with the same tempo.
Also they use Ableton Live for on the fly mixing too, so this can help fill a few gaps when setting up a completely different song/tempo on Logic. I saw them in Nottingham and when some tracks had a complete tempo change, Karl would play a sample of poetry or something like that whilst Rik was cueing up the next track.
It is a clever set up and Rik should be applauded for setting it up as it can be confusing to try and work out!!