There is nothing to "use". When the back-torque reaches a certain threshold, it operates automatically. It still allows enough back-torque to allow full engine braking without the clutch slipping, and it has to be like this, otherwise the engine would drop to off idle every time you closed the throttle. So a rough downshift will still give you a fair bit of engine braking, but the slipper clutch will cut out only the sharp impact the moment you let out the clutch with the engine speed way too low. This should cut out most of the rear wheel hopping that would otherwise happen.
I'm not 100% sure but I think on the newer versions of these clutches, you can change the threshold where it takes effect by changing some shims inside the clutch.