A severe form of engine flooding occurs when excessive liquid fuel enters the combustion chamber.
This reduces the dead volume of the combustion chamber and thus places a heavy load on the starter motor, such that it fails to turn the engine.
A hydrolock occurs when a liquid fills a combustion chamber to the point that it is impossible to turn the crankshaft without a catastrophic failure of the engine or one of its vital components.
This permits the maximum flow of air through the engine, flushing the overly rich fuel mixture out of the exhaust.
In worst cases, the excess fuel can foul spark plugs, sometimes necessitating their cleaning or replacement before the engine will start.