[Schroeder]

Ok. here we go:
First off all, you don't have to stay with your flight leader. You might hear some angry (stupid) radio calls, but it doesn't have any consequences in the game, so feel free to engage the enemy.

It seems your flying a Hurricane MK I. They don't like negative G-forces as their carburettor can't supply fuel to the engine that way (that was remedied in later versions, the Spitfire was suffering from the same problem as well btw).
What you experience when you pull the stick too hard is called a stall which can subsequently lead to a spin. A stall means that your angle of attack is too great (the angle of attack is the angle in which your wing hits the surrounding air). If it becomes too great the airflow over the wing will rip off and the wing will stall and loose it's lift. If this happens only on one wing (which happens quite often in propeller driven aircraft) then one wing still produces lift while the other doesn't. That leads to the spin. You can recover from that by centring your stick and give full counter rudder. Once the aircraft responds again pull her out of the dive GENTLY.