He's not trying to do the "block off" mod to prevent the decel popping. Tenn is attempting to gain some Hp by creating a vacuum in the crankcase which in "theory" will allow the pistons to decend quicker on the power and intake strokes and therefore increase power.
I've heard the theory before and I think it's BS. The problem is that when 1 piston is coming down, another is going up. So really the air displaced in 1 cylinder as the piston comes down is actually pushed into the cylinder with the piston going up. If you create a reduced pressure in the case then it's going to hold back the ascending pistons so any gains you might see in Hp by "sucking down" the decending pistons is gone. This is all based on the fact that you could actually create enough vacuum in the case to make a difference.
I'd like to see some hard numbers on this to back up the statement that "the engine is more efficient with the pressure drop"
Its not just the displacing of the pistons air, it is any pressure in the crankcase including pressure that leaks passed the rings. On a car(8cyl race engine) running an air pump in reverse to keep a negative crankcase pressure hp increases of 20-30 hp are not unlikely even with the pump being belt driven so there is power to be had. the rings will seal better and you can get away with a lower tension ring set therefore freeing up more hp. This setup however I dont believe will create the amount of vacuum needed to make it effective.(I ran the crankcase on my car at 12" vacuum) On the motorcycle powered sprint cars the valve covers are modified to block off the runners to the exhaust and holes are drilled to allow crankcase pressure to escape the top port on the plate, we run breathers on each plate to keep dirt out. If there was a way to gain more power the circle track guys would have found it and all the engine builders are doing it by venting it with breathers.