The Partial Seat Tube Cutout is a close-fitting curve in the seat tube that shields the leading edge of the rear wheel. The rear wheel is partially hidden from the wind, thus reducing aerodynamic drag.
As with the Extended Seat Tube Cutout, the frame must fit closely to the tire, and the seat tube cross section must have the right shape to help air flow smoothly across the seat tube and around the wheel.
Why a Partial Seat Tube Cutout, and not the Extended version? The Partial Seat Tube Cutout gives up a bit of aero to save a little weight compared to the Extended Seat Tube Cutout.