The Overdraw feat allows mystics to attempt magical effects up to three levels higher than would otherwise be possible at that level by utilizing magical energies normally impossible to control. However, doing so incurs a large chance of failure. An effect one level above normal succeeds 50% of the time (11-20 on a d20), two levels above normal has a 25% chance (16-20 on a d20), and three levels above normal has only a 10% chance (19-20 on a d20). Assuming the effect is successful, treat it as though the mystic actually spent the full amount for the effect, rather than a lesser amount with Overdraw.
For example, Yeslek the mystic wizard has 12 ranks in Transmutation and has the Overdraw feat. He can attempt a Trans 14 effect using Overdraw by spending Trans 12, with a 25% chance of success.
Later that same day, Yeslek finds himself out of Reserve, with only 2 points left in Transmutation. He wishes to create a Trans 5 effect, so he again uses Overdraw to attempt it, this time with only a 10% chance of success.
Lastly, Yeslek has 20 ranks in Divination. He can attempt a Div 21 effect using Div 20 with a 50% chance of success, without incurring the costs associated with wish-level magic.