The sideslip
The stall is a result of the angle of attack of the wing to the relative air flow (RAF). The aircraft may be pitched up in an attitude that would normally result in a stall when the RAF is along the longitudinal axis. However in the situation of the side slip the RAF is not along the longitudinal axis but from one side. In the extreme and the aircaft is slipping 90 degrees to the longitudinal axis the aircraft is actually falling but the rate of descent is resisted by drag.