"Benefit: When you cast a spell, you can decide to increase your caster level with that spell by 1,2, or 3, but you are stunned for an equal number of rounds immediately after doing so. Your increased caster level affects all level based variables of the spell, including range, area of effect, spell penetration, and the difficulty of dispelling the spell. You can exceed the normal level fixed limits of a spell with this feat, so a 9th level wizard could use Reserves of Strength to cast a fireball as a 12th level wizard and deal 12d6 fire damage.
If you are not subject to stunning effects, you instead suffer 1d6, 3d6, or 5d6 points of damage when you call upon your Reserves of Strength feat. "
DragonLance Campaign Setting Page 86
From SRD
Overchannel [Psionic]
You burn your life force to strengthen your powers.
Benefit
While manifesting a power, you can increase your effective manifester level by one, but in so doing you take 1d8 points of damage. At 8th level, you can choose to increase your effective manifester level by two, but you take 3d8 points of damage. At 15th level, you can increase your effective manifester level by three, but you take 5d8 points of damage.
The effective increase in manifester level increases the number of power points you can expend on a single power manifestation, as well as increasing all manifester level-dependent effects, such as range, duration, and overcoming power resistance.
Normal
Your manifester level is equal to your total levels in classes that manifest powers.