I don't really understand the physics of lift-off oversteer though tbh. Is it simply because the force pulling the rear around is lowered?
Im not sure either really, im sure mark can explain in detail
Lift-of-oversteer is predominately caused by weight transfer. As you let off the throttle the car's balance is shifted forward (due to lack of acc'n & engine braking taking effect). As a consequence the load on the rear wheels is significantly lightened = reduced grip. Think of is as if you were braking hard in a straight line - the back end rises and the front dips, it does the same when you let off the fun pedal, just not to the same extent.
If you do this mid bend this entails so called 'lift of oversteer' as the back of the car goes light it has a tendency to move across laterally to the direction the car is traveling in.
The reason the car oversteers is that whilst the back has been lightened, the weight has been transferred forwards, providing the front wheels with more grip, so only the back slides. (You can actually use this technique quite effectively to aid turn in just before the bend if you know what your doing)
There are other factors involved but weight transfer is the biggy. Braking mid bend is another way to induce it, but of course the weight transfer is even more exaggerated so even more oversteer happens! This is one of the reasons why cars such as the clios have such ineffective rear brakes - if they are too strong braking midbend would instantly put the back end out, so they are set 90/10 or something along those lines to let you get away with it to an extent.
Hope that makes sense-ish...