Its the filling of the head with fuel to the point that the engine cant be turned over. Liquid is essentially uncompressable so the piston cant move. Hammering on it with a starter does one of two things, the engine kicks and spins the fan loose or the rod breaks in half and the crank shaft can spin w/o the piston going anywhere.

Our engine installations are prone to flooded engines because of the way the engine is oriented, that is carb up, cylender down, its just gravity doing its thing.