Editing Yagura:SRD:Movement, Position, and Distance (section)

======Limb Damage======
Limb damage taken by the larger creature is nonlethal damage that does not contribute toward the total lethal or nonlethal damage a creature takes. Every creature has 6 Limb Locations, regardless of the structure of the creature (though it may be easier to conceive of the limb locations as coordinating to the head, torso, arms, and legs for humanoid-shaped creatures, feel free to describe it however you like). A single attack made by a smaller foe deals full and equal damage to three limb locations if the attacker is three sizes smaller, 2 limb locations if the attacker is four sizes smaller, or 1 limb location if the attacker is five sizes smaller. 

A creature's total lethal and nonlethal damage are added to the limb damage at each limb location. If this total exceeds the creature's total hit points, the limb location is disabled, imposing a -1 penalty to all d20 rolls made by the creature. If all 6 limb locations are disabled, further damage is no longer limb damage.

A smaller attacker can deal normal damage by targeting vital areas, but this is generally difficult. Attackers three or more sizes smaller than their foe can attempt to grapple the larger foe and move to a vital area with an appropriate movement (be it swim, climb, flight, etc). Attacking a vital area is done as though the attacker were two sizes larger when calculating the attack modifier, the damage modifier, and the way limb damage is dealt.

For example, a Diminutive bat attacks a Medium human commoner. The bat is 3 sizes smaller than the commoner, so the bat moves into the commoner's space, provoking an opportunity attack. The bat then bites, getting +4 to hit and -4 damage, dealing a total of 1 damage. Because the bat is 3 sizes smaller, this deals 1 limb damage to 3 of the commoner's 6 limb locations. If the bat successfully deals another 9 limb damage, 3 of the commoner's limb locations become disabled, imposing a -3 penalty to most rolls. If the bat somehow manages to keep attacking the commoner until all limb locations are disabled, the bat's next attacks deal lethal damage to the commoner.