Source: Player's Handbook II
You enjoy a stronger than normal magical bond with your familiar, granting you access to two special abilities.
Benefit: As long as your familiar is within 30 feet of you, the two of you can share the damage from a single deadly attack. Once per day, if an attack would drop your familiar to 0 or fewer hit points, you can instead choose to accept that damage. In addition, once per day, your familiar can choose to take the damage from a single attack or effect that would reduce you to C or fewer hit points. This ability applies only to attacks or effects that deal hit point damage. You or your familiar could not absorb the effect of a spell such as flesh to stone.