If you completely back off the throttle suddenly or brake heavily you will further unbalance the car!

The right thing to do is to back off slightly to try & bring the car back within its limits whilst balancing the throttle & applying corrective steering in case PSM hasn't yet been activated!

Of course thats the theory, if you have lost total control of the car & it is already spinning then you hit the brake pedal as hard as you can & hope for the best!