I saw a documentary about Federer 2-3 years ago. In one of his practices he was pushing (tossing?) very heavy balls over the net (basket ball size or bigger), first, vs one guy, then vs two, and finally against three opponents. It was a one-touch motion. Playing against three of them Federer covered the entire net. He never missed a ball or mishit it. The three guys looked tired at the end of this session. Federer was laughing merrily and talking to the coach (Paganini?) throughout the session.
Does anybody knows what this exercise trains?