I assume you're talking about the little shrouds that run from the ens of the spreaders up to a point on the mast about halfway to the cap shrouds attachment. Funny, I've never really given them any consideration at all. I've no idea how you change tension in them if it's not right.
When my boat was about 5 years old, I thought it was really time to check the rig set-up because I don't think it had ever been touched. I don't have a Loos gauge so I did it by the "2 metre stick" method which turned out to be very precise but a bit more fiddly than a gauge. In particular, I had to slacken the caps right off which was a bit of a pain. The end result was that I finished up with half a turn more on the cap shrouds than I had before. I was impressed by the almost total lack of relaxation in the rig or hull in five years. Bavaria make sturdy boats!