Slipping a pole pitch?
Imagine two magnets, sure you played with when kid. If they are oriented in same way they attract, in opposite they repel. Now, when you place both magnets in opposite orientation and leave, they will turn left or right to allign in same orientation, so this is how they slipped a pole pitch.
A generator will exert maximum torqe when rotor and stator fields are 90 degress out of phase (you can see the same with magnets), if the rotor passes beyond this limit the torque will reduce to zero at 180 degress (Torque=Tmax*sin(phi) ) and then the rotor will flip over, skip the pole with a very strong beat.