Now I want to know why you teach your daughter glove slap?
It is taught because the body likes to move in symmetry. The phrase "up together, down together" is something you'll hear often. Its fairly natural that as both arms adduct into the body with power, the glove hits the thigh. You'll also see many pitchers doing an IR movement with their glove hand that mirrors the ball hand.
Sometimes it causes problems, often it does not.
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