Oddly, the viola da gamba (viol) which I also play, uses underhand bow. But in the viol world, it's not referred to as 'German' as it was the standard. There are different 'holds' the English one is the one advocated by Christopher Simpson, there is the French Forqueray hold, which mostly differs by rotating the wrist to apply more pressure on the bow hair. With the viol, your middle finger rests on the hair and so rotating the wrist anti-clock adds pressure to the bow hair.