When driving PWM to a motor with an H-bridge, we face with the choice of how to drive the off state. The options are tri-stating the drivers and shorting the output. If the drivers are tri-stated, the inductance of the motor forces the current to flow through the body diodes from the lower FET of a leg to the upper FET of the other leg, so effectively the bus voltage minus two diode drops is appled to the motor. The current decays fast. The current flows back to the source, captured by capacitors. If the low FET is left on for the off-state, effectively shorting the motor terminals, the current recirculates and the current decays slowly and dissipates in the motor resistance. So the torque would be smoother with the slow decay.
If synchronous rectification is used, the things can get a little more complex. If the current has completely decayed and the FETs are still on, the motor goes into the generator mode and the motor terminals are shorted, so the motor is braking.
