Motor braking.

[BrianG]

Well, from what I understand (and from testing), simply shorting the motor leads puts a substantial drag on it. Try it; simply take a motor and spin the shaft. Then, short the motor outputs together and short them again. So, really, it's the motor that is absorbing the energy magnetically. I think the FETs simply apply a "controlled shunt" so there is variable braking available.
Otherwise, I would think the FETs would heat up CONSIDERABLY if they had to take that energy.

Now, if this is true (the shunting theory), I don't understand how there can be a voltage at all to get to the battery...