[caconym_]

I think it's fair to describe a forward slip as a maneuver that puts the airplane sideways to use the drag of the fuselage, which is now partially side-on to the airstream, to bleed off speed (energy, really). It's used in situations where you need to bleed altitude quickly without gaining airspeed, which would typically be the outcome of just pointing the nose down—you trade gravitational potential energy for kinetic energy.