Silly me.
Especially as I used this before. Rusty rusty rusty!
When you create an enchanted item, you must specify a physical and/or verbal trigger, anything physical, such as "when pointing the wand at someone and saying abracadabra".
As far as I can tell, this is independent of your magical resistance, or else, you'd have to lower your parma every time you want to activate a magical item.
So enchant the tile so that it casts a PeTe effect on itself, with the trigger being "when stepped on it".
Actually, scratch that. "When someone steps on it without saying abracadabra"