there is a lot of misunderstanding among the pilots with regards the two types of slips.
A Side Slip is used to correct a crab condition and to line up the aircraft with the runway centerline during a X-wind landing.
You bank into the wind and apply rudder to keep the longitudinal axis of the airplane aligned with the runway centerline.
A forward Slip is to loose altitude if you are high on final. slight bank not more than 5 degrees and apply full rudder in the opposite direction. a perfect forward slip would have the aircraft longitudinal axis NOT aligned with runway extended centerline (or the relative flight path). This will cause the side of the fuselage to meet the relative airflow and cause high parasite drag and hence the loss of altitude.
They both serve a different purpose.