It's when Dva climbs back in the mech. When you fall out, she still has 150. If you're damaged, say you get down to 100 hp, climb back into the mech at that 50 hp will be healed so that next time when you fall out of the mech again you're fully healed at 150 health.
Im pretty sure that if you defence matrix a mercy pistol shot, it travels to a different parallel reality whereas damage is healing and healing is damage so mercy is actually healing with damage thus rippling the space time continuum and causing a Mercy's beam to eject through your defence matrix at your team at the speed of light resulting in a third dimensional paradox whereas dvas defence matrix conforms to a two way system of both negating damage and doing healing at the same instant but can only happen once in one quintillion matches. That's my theory anyway