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There are a couple of ways to deal with a high backhand
The first and obvious way is to hit the ball with a suitable grip, the extreme eastern,(aka semi western) backhand grip, which places the palm of the hand behind the the handle, allows you to hit the ball with authority and spin from any height, unless oyu have some technical deficiency you are not going to struggle with high balls, it just won't be an issue.
The minus with the extreme grip is that you cannot slice with the same grip.
If you use one of the more classic grips, you are going to struggle with the high ball, or, rather, you are going to struggle to hit the high ball with topspin.
The only uncompromised way of hitting a high ball with such a grip is to hit the ball with a very slight high to low swingpath.Ivan Lendl was the absolute king of this, He just used to blast winners by people with this shot with no noticable dip in power.I can't seem to find a clip of this shot just yet but I'll explain it to you.
Basically, at shoulder height, it is anatomically just 'wrong' to hit a topspin shot with a classic grip, (maybe even lower than shoulder height for very conservative grip users, eg, tim henman), the only way the racquet face can be orientated correctly is if the racquet head is above the hand, swinging low to high with the racque head above the hand with great effort puts a lot of strain on the elbow, and it's unnatural and requires a lot of effort to stop the hand and wrist naturally folding back, which stiffens up the arm, which results in a weak shot.
The reason a slice is easy on a high ball is becasue as you are swinging high to low for backspin, the racquet head is naturally above the hand and orientated correctly.The key is to flatten out the slice the same way you flatten out a topspin gruondstroke, (think of Safins flattened out topspin backhand for example), where the ball is being hit with a certain amount of rpm's of spin, but not enough to exert any effect on the ball.In the case of Safin's flattened out tposin backhand for eg, there is topspin on the ball, but nowhere near enough to influence the trajectory of the ball/ pull the ball down, at all.In the case of the flattened out slice, the same principle applies, you swing on as flat or as horizonatal a trajectory as possible while still maintaining some low to high.The height of the ball you are striking means it naturally has decent net clearence.This gives you the freedom to go for the flat winner, or hit with slightly more slice and less pace for more safety.
People nowadays are always amazed when they hear how great Ken Rosewall's slice backhand was supposed to have been, What people don't understand was that Rosewall drove that slice backhand as hard as could be.
I believe the extreme eastern backhand grip is the way to go, if you're going with the classic grips, hit the ball as described above.