A H-Bridge for use as a motor driver switches direction by comparing an input voltage on a signal line to a reference voltage generated by a voltage divider .
Importantly, there is neglible current draw from the signal line.
The voltage divider resistors can be varied to match the input signal line voltage thresholds.
This modern variant uses mosfets in the h-bridge but older variants use transistors that have an undesirable characteristic of generating alot of heat for comparatively little current transfer.
Back EMF protection is provided by the optional pair of inductors and the capacitor across the load that is represented by a resistor. Using transistors instead of mosfets would also require the introduction of diodes to clamp the back EMF and protect the transistors (mosfets usually have clamping diodes built in).
In practice an enable signal can be provided by controlling the supply voltage (typically the ground) to a dual comparator chip that would house the pair of comparitors.
