If a mystery present is Amped when opened, it remains in your inventory as a regular, identified version. With Amped-this-level being a factor, it provides an infinite loop exploit that allows unlimited uses of key items by simply dropping all other items.

'but how would you know what is in a mystery present ahead of time? how can this be an exploit?'

Hold a present you value: rocket skates, hi tops, wings, jump 2 ship piece, etc, then when you see a Man In Black, drop all other items. Let him turn it into a mystery present, and you know its contents & can use it every level.