When and why a forward slip approach is used during an approach.

Bottom line

The value of the forward slip maneuver explains its inclusion as a task in the Private Pilot Airman Certification Standards (ACS). In an engine-failure emergency, a pilot may initiate a descent using a forward slip much more quickly than by deploying flaps. To reduce the descent, the pilot can remove the slip without penalty, whereas retracting flaps on an approach could lead to an unwanted loss of altitude.