A power stroke is, in general, the stroke of a cyclic motor which generates force. It is used in describing mechanical engines and molecular motors such as ATP synthase. Many types of motors can be simply described by first, intake stroke (intake of fuel, e.g. gasoline, ATP, etc.) then power stroke and last exhaust stroke (exhaustion of what's left of the fuel which is now in a low energy state), possibly with some steps inbetween such as the compression stroke in four-stroke cycle engines and then repeating the cycle.
In muscles, the power stroke is the stage of muscle contraction when the cross-bridge (connecting the actin in the thin filament to the myosin in the thick filament ) moves towards the H-zone , thus causing the muscle fiber to contract. The energy for this process comes from ATP present in the myosin prior to contraction.
In sports, too, often a swing with a lot of force will be called a power stroke. For example, this is used in canoeing to describe a powerful motion with a paddle.
