When you are singing Renaissance polyphony, your part follows its own melodic line (no such thing as melody-over-chords in those days). Since it goes off on its own, you have to keep the shaping of your own line correct - a lot of us wind up using "body english" to keep it going.
Somebody on one of these videos referred to it as "early music Tai Chi". It really is, and it really works (at least for me).
Which is why choir lofts are in the back of the church. Nobody sees me gesticulating and shifting from foot to foot except the priest and the deacon.
I was wondering about that.