The HP VT series of engines were a rotary valve 4 stroke engine. There is a gear drive and a driveshaft to the overhead rotary valve. As the rotary valve rotates it opens the intake and the exhaust ports respectively. It does not use poppet valves like the conventional 4 stroke engines use. The rotary valve design doesn't have problems with over-revving the engine as there are no poppet valves, no camshaft, rocker arms, nor valvesprings in it to cause the valves to float. They call the rotary valve in the HP VT engine a Aspin Valve. It is a gear driven conical valve with a opening that as it rotates it exposes the intake port, then the glow plug, and then the exhaust port.
They were talking about it in this thread too http://www.rcgroups.com/forums/showt...413&page=3