This is a fascinating disucssion.
I guess most of the slappers must have just seen the title of the thread and shrugged it off as, 'to each his own', otherwise by now we would have seem a comment like 'my thumb never rests'
jazzy - I'm with you on this, better to just let the fingers do what they want, because they're way smarter than I am. Sometime last year (before my Alembic arrived) I noticed one evening that the little finger on my right hand was resting over the bridge saddle of the B string. I thought, that's weird, just ignored it and went on playing. But then I noticed it again a couple nights later, and became more curious about why it was doing that.
So I experimented a little on the B, with my little finger in various positions, and it turned out that it was doing a very subtle damping - somewhat like a compressor, perhaps. If it wasn't there, the B was a little soft and flabby sounding in comparison to the rest of the strings. But with a very light touch - and in a very precise location, within a range of less than 1/16 of an inch - it tightened up the tone very nicely.
At some later point, I noticed it would occasionally sneak itself in between the body and the string, sort of like what you described for your thumb. Not really pressed in there, but positioned just so that on a large excursion, it would damp the string a little, but with more gentle playing it wouldn't be in contact.
It's just amazing to me that 'my finger' figured this out on its own, with no conscious thought whatsoever.
Getting back to the thumb, it seems to me people are talking about at least two or three different kinds of 'floating', and it's worth being clear about that.
One case is when you want to move your plucking position up or down along the length of the strings, to get a different sound. Several people here mention this, though most of them are still looking for places to rest their thumb as they shift this way (end of the fingerboard, neck pickup, etc.)
Another standard case is when playing across the strings, where one suggested approach is to always rest the thumb one string above the one you are playing. That's useful, but I'm just not good enough to reposition my thumb that frequently... And assuming your wrist or arm is somehow braced against the body, I don't understand why you need to also brace your thumb - but that's just me, and whatever works for you is fine.
I kind of think of these first two cases as drifting or shifting, more than 'floating'. In my case, my right thumb is never braced anywhere. It just sort of hangs in a relaxed postion, more or less as it would if my hand were hanging down by my side, and laying perpendicular across the strings. But it stays in light contact with the fatter strings, and with my fingers angled back towards the bridge about 30 degrees, it always seems to be resting lightly on the strings below where I'm playing. Meanwhile, my left is naturally damping any strings above the one I'm playing, so both ends are covered - without thinking about it.
Sometimes the thumb decides all by itself whether to let a lower string sound a bit longer, or damp out immediately, or maybe do a little primitive percussive thing.
All this is fine by me. One of the great things about my Alembic is that the B is so solid that my little finger is free to explore new territory - it's not really plucking yet, but my third finger has become a lot more active, and I think the pinky is doing some new stuff sometimes (but I'm trying not to pay attention to it).
-Bob
