It's simple if you would just think about it.

One cylinder fills with water enough to cause a high pressure situation, which holds the valves shut, and places pressure on the cams, enough pressure that the camshafts will slow drastically in comparison with the crank. Result? The weight of the flywheel tries to force the engine to roll over, the cams don't want to move, you have one cylinder with valves open while the original cylinder is closed up and unable to move, the belt stretches or skips, and the piston immitates mike and tina with the valves. Anything over 1500 RPM would be sufficient speed on most engines to make this scenario plausible. Valves hitting pistons don't bend them, pistons hitting valves bends valves. Also during this type of action is when piston ring lands break and water can find it's way into the crank case.