A Hydra Lock is used to keep the Greek Mythology serpent contained in some form of enclosure.
Hydro-Lock is short for hydraulic lock. In an automobile, it's a condition in which a cylinder becomes filled with a fluid, preventing a piston from moving within. It can happen if something within the fuel system allows fuel to flow into the cylinder after the engine is stopped.
It was a common problem on one of my old Craftsman mowers with a Briggs-Stratton engine. The needle valve and seat were bad about leaking and the fuel would continue to flow into the cylinder. Because the liquid wouldn't compress when trying to start, it would hydraulicly lock the engine, preventing it from turning. In later years, they installed a small solenoid on the bottom of the bowl that was supposed to close off the fuel when the ignition was turned off.
In liquid cooled engines, it also happen when there's a gasket leak that lets coolant seep into the cylinder.
