Specification: Type: Tennis Self-study Device Quantity: 1PC Material: Plastic+Rubber Color: As shown Elastic line length: 3.8~4 meters Base Size: 20*14*5.5cm/7.86*5.50*2.16inch Package include: 1*Tennis trainer Fill-n-Drill is the perfect portable tennis training aid. Simply hit the ball and it comes back to you! Fill the base with water or sand nd it instantly becomes heavy enough to hold the unit in place. Non-skid pads underneath help keep it stable Unwrap the cord, hit the ball as hard as you want, and the ball bounces back to you. -The harder you hit the faster the ball is rebounded to you Rubber cord is 15 feet long and stretches to 30 feet for hard hit balls. To shorten the rebound, wrap some of the cord around the base to your desired length A great device to practice your strokes, develop a faster swing speed, or simply warm up