Blood: yeah, I got that, but I didn't get the part where Dumbledore says, "Lily's protection inside both of you! He tethered you to life while he lives!" (709)

I don't get that. If he just means that Voldemort sustains Lily's protection so that Harry can't die as long as Voldemort lives--why doesn't Harry die when Voldie dies? Harry asks that next but Dumbly and JKR neatly avoid it.