Ideally, you want top spin on ground balls and back spin on fly balls.
If a hitter is trying to match the plane of his/her swing to the plane of the pitch as some teach, then usually those things are accomplished based on how the hitter has timed a particular pitch. All hitters are trying to hit the ball square on the nose(which many times results in a ball knuckling off of the bat)but of course those darn pitchers are always trying to mess up a hitters timing
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Swinging down for back spin is a very difficult thing to do and to actually accomplish. Prove this to yourself by trying to hit fly balls to your teamates for outfield practice. With each swing toss the ball for yourself and then try to swing down in order to create backspin on the flyballs you hit. If you actually hit many(if any) flyballs for them, I would be greatly surprised.