The propeller disk theorem sais that thrust is caused by Newtonian principles. The action is to cause a pressure difference between front and back of the disk, the reaction is to accelerate the air passing through the disk. The pressure difference is what causes the thrust.

That is in a perfect world. We live in a non-perfect universe, so the pressure difference will cause air to flow from the high pressure region to the low pressure region, where the air flow path is around the outer disk rim, but also through the inner hole where the prop turns too slow to sustain any difference.
This pressure difference is the disk load. Flow around the disk (tip vortex) will increase with disk load, and decrease the specific thrust per amount of power applied to the prop.