Leveling up a Pokemon when you do not have or do not want to use a Revive will give a Pokemon 2 more max hp. This means the fainted Pokemon will now be revived and have 2 hp.
Alternate Revive Method
from PoGo Admin | 1st Aug 2016
Leveling up a Pokemon when you do not have or do not want to use a Revive will give a Pokemon 2 more max hp. This means the fainted Pokemon will now be revived and have 2 hp.