I have a query related to voldemort's dearh and harry potter's survival?

well....in the last part where harry goes into aragog's(the spider)living place where voldemort calls him,voldemort uses the killing curse.but harry just collapses and reaches the kings cross station and meets voldemort!!!how is it possible??and also though his mother's sacrifice which protected him had ended,still he manages to survive.......its a tad too confusing.....