Overcharge is quite useful for both defense and offense.
Defensively, it allows for almost instant repairs and upgrades on the team's defenses in a pinch. Any Apprentice towers recently destroyed can be quickly replaced by the user.
Offensively, Overcharge lets the Apprentice to cast Mana Bomb and heal himself much faster, even in the middle of combat.
After deactivation, there is a three-second cool down before the user can Overcharge again.
Leveling And Costs
It costs 8 mana to activate Overcharge and 8 mana per second to maintain which will slowly increase in cost the longer it is used. The more points invested into the Overcharge skill, the greater the increase in casting rate and reduction in the ongoing cost.