Now since the base-emitter junction of a bipolar transistor is essentially a diode we can say that the drop between the base and emitter of the upper transistor will also be . This has two implications. Firstly, when the output voltages will be zero. The second implication is that as we adjust to produce a signal, the emitter voltages will both tend to follow it around. In effect, when we wish to produce an positive voltage the upper transistor conducts and provided just enough current to supply the load.
None is wasted in the lower transistor.
When we wish to produce a negative voltage the lower transistor conducts and draws the required current through the load with none coming from the upper transistor. Again this means the system is highly efficient in power terms. When power efficiency is the main requirement then Class B is very useful. However to work as advertised it requires the voltage drops across the diodes and the base-emitter junctions of the transistors to all be exactly the same. In practice this is impossible for various reasons.