Most of the tracks in this collection are largely improvisation: just here we're looking at

Improvising a prelude

During renaissance & baroque times it was common to improvise a prelude before you got started in the performance proper; one of the reasons was to get the player and the listener into the right mood - and in the same mood as one another: another point was to check the physical, the harmonic, tuning within the instrument:

• you tune first, then you test it a bit ...
• you can use a scale - two scales - three scales ...

There are plenty of written examples, though it was always improvised at first: the teaching books from earlier times describe rather clearly how you do it.

These versions useQuickTime - there are YouTube version here