Slow Decay
The mode in which the motor terminals are shorted during the off phase of the PWM so that the current circulates inside the bridge. Because the current falls slowly the ripple stays small and the torque is smoother; the motor shows a braking behaviour in this phase.
During the on phase of the PWM the supply voltage is applied to the motor and the current rises. In the off phase the current needs a path to continue. In slow decay this path is inside the bridge itself: the two low-side switches (or the two high-side switches) are turned on together, the motor terminals are shorted and the current circulates in this loop.
Because only the winding resistance and the switch resistance are in the loop, the current falls very slowly. The result is low current ripple, low torque oscillation and quieter running. At the same time the motor behaves like a short-circuited generator in this phase; that is why slow decay also produces a mild braking effect.
Its limit is fast response. When the current has to be brought down quickly (e.g. a sudden direction change or a hard entry into the current limit) slow decay is not enough and fast decay or a mixed mode is preferred. In practice many drivers use a blend of the two: part of the period in slow decay, the rest in fast decay.
The context in which this term is used: DC Motor Driver Selection Guide