Re: Have a question about two-handed backhand?
One thing that hasn't been mentioned in terms of the actual swing and generating power is making sure the arms are relaxed. One of the biggest mistakes people make on the two-hander is a tendency to firm up on the shot a bit like you might on any one-handed swing. Many people do this either when trying to force the ball over or in feeling constrained by using both hands.
But you have two arms to handle any incoming ball, and you've got that plus your core and legs (as everyone has mentioned) to generate power. It's important to be as relaxed as possible with the arms in order to optimize your potential for swing speed.
It's a basic thing that a lot of people fail to do. You see a lot of stiff two-handers, people choking up on the handle or trying to force the shot, and they just don't get much out of it.