The valves will still open and close as they should so there will be air movement. The piston will compress air as it normally would and that energy will be mostly returned on the piston's return trip back down (it's like jumping on one of those big air-filled balls). And you don't want the cylinders to cool because it could create problems such as uneven wear, which is why I might want to alternate which bank is being used while the engine is running to make sure temps stay close and wear is balanced.