lifting the right hand pinkie
I see a lot of professionals do this when they're playing tremolo or some sort of staccato, but it's never been taught to me (my teacher even does this). I'm sure this is something they're aware of, so what is the advantage of doing it?
The advantage is that your lifting your pinkie means that your wrist is turning counter-clockwise into its most efficient position and quick movements (like staccato, tremolo, bowed ornaments in fiddle music) feel effortless. Fiddlers will say that bowing is done with the thumb and the first and second finger of your hand, which isn't true for people wanting to control the lift of the bow or play closer to the frog, but it gives you an idea of how the quickest movements are done.
I don't think it's necessary you "learn" this. Just do as your hand feels comfortable. Maybe it's also a matter of one's individual hand anatomy, I don't know. I never "learned", but e.g. an upbow staccato on the upper half of the bow is almost impossible for me with the pinky in place, and even more if an upbow martele is required. I need an amount of pronation which inevitably makes it lift at least a bit. I'd even prefer it otherwise and worked on this, because I think it would give me more control, but it wouldn't work...
Reality; The fourth finger is shorter than the third. Depending of your individual anatomy, bow-hold, playing angles, at some point on a down-bow stroke, the fourth will extend until is straight. If you insist on it being straight and rigid on the stick, the right hand will be in danger of being locked, inflexible, loosing control, tone quality and even volume. So you let it come off the stick. The tremelo at the tip is a perfect example. Some folk fiddlers, who never bounce the bow, and rarely play near he frog, will have the little finger off all the time.