Floating or Double Clutching

Floating gears is one of those things that comes with time that's kinda accidental. One day you just do it and you realize "Oh crap I just floated!" At least that's how it was for me. My cdl instructor told me there's no need for more than a couple fingers on your shifter. If you've got it in a death grip you're going to do more harm than good.

As far as the bit about the gears spinning and what not, when you shift at the right rpm's, the gears are spinning at the same speed. When you're grinding gears, that's when you're scraping gears against one another. When you float gears, you're just slipping right from one gear to the next. When your rpm's are right, they're spinning at the same speed and they just drop right in.