Its used by below avg hitters to try and be avg ghitters cause they need every advatage possible. By the rule books just a drop or quick spray is illegal. If u want to get technical its not allowed at all. Its concidered altering the bat. Ive seen umps allow alot and ive seen em not allow any. Just like ive seen umps throw bats out for a sticker that has 0 effect on hitting a ball.