Sauen

The diagrams you're seeing are from reed-valve two stroke engines, this indeed prevents the fuel/air mixture from backflowing. Such engines were used in early chainsaws, but nowadays most chainsaw engines do not use such a valve. They rely on the momentum of the air/fuel mixture moving through the case and cylinder to pull air and thus fuel into the case. Indeed, some of this can be blown back if the intake duration is too much.