As I understand it...

Each Realtrack is a collection of samples of a real instrument. This collection comprises audio recordings of the same instrument playing in different keys/different chords/patterns all at the same tempo. It also includes recordings of the instrument playing holds and shots etc

BIAB's function when playing Realtracks is as a VERY sophisticated sample player. It looks at the tempo you have entered, the chords entered, part markers and any rests/holds etc. It then selects one of the samples in the realtrack that is suitable based upon these variables, and speeds it up or slows it down to match the tempo (the sample speed is changed so the instrument playing sounds faster or slower than the recorded tempo, but the pitch of the notes stays the same).

That is the clever bit!

BIAB also looks at the preceding and following chords when choosing which sample to use, making it a pretty impressive piece of engineering.

The Realtracks also contain variations for each chord/pattern, so when you regenerate a track BIAB may choose a different sample, still appropriate for the song bar but different to the previous generation. Hence it's always worth generating a few 'takes' of each instrument to see which you like best

I would imagine that the Realtracks are built up using bars as a basic measure. If you look at the tutorial on this site on how to create your own Real DRUMS, it will give you an idea of the logic. However, realtracks also accomodate key and chord changes which is what makes them so impressive and almost impossible to make yourself!

Hope this helps

regards
Martin