The potion should give a limited amount of time to use the gear. If you're still using over-leveled gear when a new quest triggers a combat sequence, it will be automatically unequipped.
I've never actually used it, myself. It was introduced after one of the patches rebalanced weapons and armor: players might be in the middle of a quest or something, and after the patch, they'd find they couldn't actually use the gear they had equipped. (For now, it functions more or less as a minor cheat. If you began playing the game in the latest version, there will be no issue with the gear you find.)