Cavitation
You will experience the phenomenon of "cavitation" when you pull the gas lever to the handle (WOT) and the engine revs, but your do goes no where.
I experienced the same thing in 1993 when my wife sucked up a rope on the River. Didn't know what the hell happened, however, when she hit the gas and went no where, I knew something was wrong.
When I saw the rope tangled up on the drive shaft, I thought I was screwed, but, some patience and a knife got rod of the rope.