Video: Watch Flying Robots Cooperate, Throw and Catch Balls | Robohub | Heron |

The quadrocopters are attached to an elastic net. The net allows them to catch the ball by moving such that they span the net under the falling ball. Since the net is elastic, the quadrocopters can not just catch but also toss the ball back into the air by rapidly accelerating outwards, thereby stretching the net and launching the ball from it.

Via The Robot Launch Pad