Question: When should I hit flat on my backhand?
Answer: When you hit flat, none of the energy of your swing goes into creating spin, so all of it goes into driving the ball forward. You get the most raw power when you hit flat, but without topspin to push the ball down into the court, your margin over the net on hard, flat shots is small, making them risky. Compensating somewhat for that risk, though, is the greater simplicity of executing a flat shot: the longer alignment between your string bed and the ball makes your contact more reliable. This certainty of contact makes less aggressive flat shots, typically with a short swing, sometimes a good choice in defensive situations where you can't set up properly to execute the more difficult timing of a spin.
You can hit flat one-handed backhands on any ball you can reach, although slices can be easier on extremely low balls.
Most players can hit a flat, two-handed backhand comfortably on balls roughly from mid-thigh to a little above head height, although at the high end of this range, hitting hard is difficult. On balls below and above this range, a one-hander is usually easier.
