Fast Decay
The mode in which, during the off phase of the PWM, the motor current is decayed towards the supply source under a reversed voltage. The current falls quickly; this gives fast current control but noticeably increases the current ripple and the heating that goes with it compared with slow decay.
In fast decay the bridge, instead of shorting the motor terminals during the off phase, connects them to the supply line with reversed polarity. The current now tries to flow not only against the winding resistance but against the whole supply voltage; this is why it falls with a much steeper slope. Part of the current returns to the source and charges the bus capacitor.
Its advantage is speed. In cases where the current has to follow the reference quickly, for example on a hard entry into the current limit or during a fast direction change, fast decay becomes necessary. Its disadvantage is that for the same average current the ripple is much larger; this means extra heating and more noise on both the winding and the switch side.
That is why most drivers use mixed decay: fast decay is applied during the first part of the off phase and slow decay during the rest. This preserves the control speed while keeping the ripple at a reasonable level. On the user side this choice is generally invisible; its result is felt in the sound of the motor and the temperature of the driver.
The context in which this term is used: DC Motor Driver Selection Guide