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if you could see your opponents cards and would play the hand the same way, you benifit. If you would have played it differently had you seen his cards, your opponent benefits. and on the other side, if your opponent can see your cards and would have played HIS hand the same, he benifits, if he would have played it differnetly you benefit.

I dunno if i explained it good, but by showing your hand you let your opponent it play it perfectly. (folding) therefor, they benifit.