You're forced to hit a half volley when the ball lands at roughly the same depth as your feet, too short for you to hit a strong volley and too far back for you to hit a full groundstroke. You end up hitting the ball just as it's coming up off the court.
One nickname for the half volley is the "Ba-Boom" shot. The "Boom" part is a little misleading, given that it's usually a fairly gentle shot, but if you say "Ba-Boom," you'll imitate the proper half-volley timing between the bounce and the hit.
Timing is the key to a good half volley. You want to meet the ball as near to its bounce as possible, which means picking it up very close to the ground.
The ball has an upward path and a good amount of energy as it comes up off the court, so a shorter swing that makes precise timing easier also helps to prevent overhitting the ball.
On the forehand side, the Eastern grip is usually easiest for a half volley, but a Semi-Western or Continental grip can work too with practice at correcting the racquet angle.
If you hit a one-handed backhand, stick with it for the half volley, and try either an Eastern backhand or Continental grip. The Eastern has less tendency to tilt upward too much.
Two-handed backhanders split pretty evenly between those who do better with one hand versus two. With one hand, it's easier to get down low, but with two, it's easier to meet the ball farther back when you need to. Experiment to find out which choice suits you better. Knowing how to do both can come in handy. For the two-handed option, use your usual grip combination.
