The grass whip is a long-handled tool, generally made with a wooden grip and a metal shaft. The shaft extends straight out from the grip for several inches before bending downward. The angle of this bend varies depending on the manufacturer. The shaft terminates in a blade, fixed to it at such an angle that the tip points diagonally upward when the grip is held horizontally.
A grass whip's blade is long and rectangular, with a flat tip. Most models have a hole punched in the blade near the tip to allow the tool to hang from a nail in your tool shed. The long sides taper outward at both ends, with a slight pinch in the middle, and are both serrated along nearly their entire length. This allows the grass whip to cut no matter how you swing it.
The grass whip is meant to be held firmly in both hands, in much the same way that you hold a golf club. Swing it back and forth, from left to right, in a smooth arc with the blade at the desired height of your grass in the middle of the swing. Walk slowly forward while swinging to clear a swath of grass. Turn around once you reach the end of the area to be cleared and continue back, cutting the next section of grass.
Golf experts Bill Melhorn and Shawn Clement both recommend using a grass whip as a tool to improve your golf swing while cutting grass. The ideal golf swing is very similar to the method most people naturally use when swinging a grass whip, while the exercise derived from using it strengthens the muscles used when golfing.