The amplifier is efficient in its job because each transistor is only on during it's respective swing (+ for the NPN and - for the PNP).
This is also a good way of visualizing the difference between NPN and PNP transistors -
The NPN conducts current from collector to emitter when Vbe is greater than 0.7V and a small base current is being sunk.
The PNP conducts current from the emitter to collector when Veb is greater than 0.7V. The base must be pulled 0.7V lower than th emitter.
