I think I found the Sakura learns torture fic that you were thinking of. It is (if I'm not mistaken) part of a series by rayemars. On her livejournal, she lumps it in with her thirty kisses set, but it's kind of supposed to take place before her story "Ripple Effect". The website is http://rayemars.livejournal.com/135200.html#cutid3.
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