The work done inside an engine is not done by compression... that is only something which effects the amount of power/air/fuel that is in the cylinder able to be burned... all the work is done on the downward movement of the piston in the power stroke... the fact that the timing on non diesel engines may be before TDC is a reflection of both the time it takes the fuel/air mixture to ignite completely... and the shorter relative stroke... giving less "working" distance.

For several degrees of crankshaft rotation there is a very small movement of the piston...but the placement of the crank, so that the power is applied in the direction of rotation desired is accomplished during that time....