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When air comes against a propeller, the shape of the blades causes a difference in air pressure. Propellers have a twist in the middle, so when the propeller spins, that twist causes the air in front of the blades to spin faster, decreasing the air pressure. The air behind the propeller is not affected as much, so there is higher pressure air behind the propellor, resulting in a bigger push from behind, generating thrust.

This means that if the propeller faces oncomming air, the spin of its blades will be enough to lower that pressure in front of the propeller and push the plane forward. This difference in pressure is enough to produce thrust, no matter what the pressure was before!