The roundoff as a gymnastic technique is to turn horizontal speed into vertical speed (to jump higher); it is also used effectively to turn forward momentum from a run into backwards momentum, giving speed and power to backwards moves such as flips and somersaults.
In a round-off, you land with your two feet together on the ground, facing the direction you arrived from. This is achieved by twisting the hands and shoulders as the hands are placed on the ground. The two hands are generally placed down one after the other, so that the first hand is twisted by 90 degrees and the second hand by almost 180 degrees from the direction of travel.
This technique is seen in men's and women's floor gymnastics. The round off sometimes initiates a row of back handsprings, flips and twists (called tumbling).
Round-offs are used by most acrobatic sports, including certain types of dancing and cheerleading.