Much like a balloon loosing the gas inside of it over time, triggering this on the target would decrease the max mana of the target by a percentage, while slowly releasing unused mana into the universe. Mana Tap would decrease the targets release of mana, but not prevent it. Non-triggered players/mobs with mana_tap in the same room would get an additional small amount of mana replenished of people with mana tap in the room due to the access amount of "spiritual" energy in the area.

But back to the psionic skill, this is an awesome idea, and I really think it should be implemented. I'm going to go post this idea for Mage :)