A flyback diode is used to protect electronics from the voltage spikes generated when an inductive load such as a motor, is disconnected. Cutting power to a motor generates a voltage spike of the opposite polarity, and a flyback diode absorbs this, protecting the driving electronics.
When a motor or other inductive load is driven unidirectionally, connecting a diode in reverse across the load, as shown on the left, is sufficient. When a load needs to be driven in either direction, a diode bridge can be formed for each pole of the load, as shown on the right.