In a good slip there is an balance between the turning force generated by the bank of the wing and the opposite rudder force which you apply to keep going straight.
Due to the fact that the relative wind blows against the fuselage you increase the drag. To overcome this you can either increase the speed or the angle of attack.
When you increase the speed there will come a point where the force the banked wings will become greater than the opposite rudder can compensate for and the aircraft will 'run out of the slip'
When you increase the angle of attack by pulling on the stick the force of the banked wing will reduce and the rudder (if continued to be applied full) will 'win' and the aircraft will start to yaw towards the direction of the applied rudder.
FD