A simple example would be an inline four engine. During combustion the energy released will push down on the 2 of the pistons. This action causes the crankshaft to turn 1/2 way and force the 2 pistons that were down to go up. These 2 pistons were not generating any power at that moment. Combustion of these two pistions will then turn the crankshaft the remaining 1/2 and complete a full revolution. Does that sound about right?