Welcome to Librarium Online!
I don't play necrons but have been hearing a lot about phasing out, could someone please explain this to me???
The short answer is when a necron army gets to 25% of starting strength the army teleports off the battlefield, thus meaning necrons instantly lose.
Not all the necron models apply to phase out (i.e they don't count ot the phase out number).
We're not allowed to quote rules here as it's against forum rules. Why not ask someone in person that plays Necrons or go to your local store and check out the codex?
I do not criticize. I do not offend. All I offer is my opinion which means only as much as you make it.