What's the difference between feathering, and flapping in a helicopter?
Feathering

The collective pitch control, or collective lever, is normally located on the left side of the pilot's seat with an adjustable friction control to prevent inadvertent movement. The collective changes the pitch angle of all the main rotor blades collectively (i.e., all at the same time) and independent of their position.

That's what you referred to as feathering, notice it says &quot;independent of their position&quot;. It's for collectively increasing or decreasing of the lift on all blades—the engine will keep the RPM constant.

Using the collective in level flight would cause a climb or descent, while with the helicopter pitched forward an increase in total lift would produce an acceleration together with a given amount of ascent.
— Wikipedia


Flapping
Flapping is position dependent as explained here. Flapping is not pilot controlled. It's to counter the dissymmetry of lift.