As the others said, Float just vents both ends of the cylinder(s) to tank (the transmission). Just think of it as suddenly not having lift cylinders on the boom. Handy for skimming or back dragging, or as another put it, getting that bucket down QUICK if something is going wrong. It may be a rough ride when it gets to the ground, but that's a whole lot better than looking at it upside down or sideways.
Until my LX, I'd never run a FEL with Float. A little trick I use is get the bucket close to the ground and just before it gets to the ground, maybe a couple inches before, I give the stick a shove into float while the boom is still coming down. Not much of a bump then. It's a timing thing that takes practice, but the more you do it, the better you get at it. It's even better if you have the bucket in a full dump position, particularly with a tooth bar that softens the impact of the bucket edge.