Ash,
It's where the aircraft is physically inverted, but still nose down. To the pilot, it appears that he is spinning upwards towards the ground. Many aircraft will come out of the spin if the controls are released but if not, the stick is moved BACK, rather than the forward stick needed to recover from a fully developed, normal erect spin. The (piston) engine is likely to stop if the aircraft doesn't have an inverted fuel supply, which makes it more exciting, but may start again all by itself as the aircraft recovers.