The Flatten Brush moves vertices towards the target plane. Strength controls the amount of displacement
The target plane position and orientation is set based on the initial face that is clicked to start the brush stroke. The target plane does not change until the mouse button is released.
Hold SHIFT to constrain the target plane to be horizontal. The elevation of the plane is set based on the point that is clicked at the start of the brush stroke.
When strength is inverted, vertices move away from the target plane rather than towards it.