I think it was fairly clear. At the end of book four when Voldemort got his new body, he made it out of Harry's blood. So the magical protection of Harry's mom's sacrifice is attached to both of their. So the killing curse gets only the foreign soul (Voldy's last Horcrux), and Voldemort and Harry continue to live, without Harry's mom's protection, and Harry without the fragment of Voldemort's soul.
So Voldemort has only his body and the snake, while Harry has only his own.. cleverness.. and the wand ownership magic on his side. Harry's sacrifice for the entirety of the school protected them from Voldemort's spells, like his mother's sacrifice protected Harry from Voldemort. So only Harry, now, is vulnerable to Voldemort, but because the Elder Wand is loyal not to Voldemort, but to Malfoy, who is the former owner of the wand Harry is using, Harry is able to disarm Voldemort and have Voldemort's killing curse backfire on him.
Wait, so I guess it was a bit confusing.