A leg drag turn is just what it says, you drag one leg during the turn. You drag the leg that is to the inside of the turn, so you swap feet for opposite turns. Turn right, drag the right, etc.
Depending on how fast you're going, you put one foot in the middle of the tray (or, if you're going fast enough, more towards the side you're turning toward, even right along the edge of the tray if you're really rippin' it) and drag the other leg lightly over the water. Flatten your foot out behind you so it's less drag/turbulence. You have to have good boots or be barefoot, because this will rip most shoes right off.
I haven't tried this on my FX-1, but have on the SN. It definitely takes some nerve to try this, you have to be moving pretty good for it to work, but you can make a sharper turn and carry more speed with a leg drag on narrow boats. I don't know if anybody dose these on SXR's.
The best I've personally seen at leg-dragging turns was Victor Sheldon. That dude ripped. I'm sure there's plenty of others, too, but he's the first that comes to mind.