Just a technical terminology note:
To lose altitude as rapidly as possible, without changing direction, one uses a
forward slip. Useful even with normal engine power, if one must make a no-flaps landing.
A
side slip is used to counter drift from a crosswind in a normal landing.
http://avstop.com/ac/flighttrainghandbook/imagetck.jpg
They are more or less the same thing, using the same technique - crossed rudder and aileron controls. The difference is that in a sideslip, the nose remains pointed in the direction of travel (runway centerline), whereas with the forward slip, the nose is pointed off to the side of the direction of travel, presenting the side of the aircraft to the slipstream, thus adding drag to allow a steeper descent.
Or put another way, in a forward slip, the
slip is in the direction of travel, while in a side slip, the slip is in the direction of the wind.
Of course, if one is trying to make a no-flaps landing or come down fast, and ALSO counter a x-wind, one can make a slip "in between" a pure forward or side slip.
I've made several forward slips to an actual landing in primary and recurrent training, in response to the instructor/check-pilot putting his hand over the flap lever and saying "You have no flaps - how are you going to land this thing?"