Find the point at the leg hemline where the line begins to curve.
Draw a straight line from this point to the single notch on the back panel seam.
Cut this line beginning at the back panel seam, ending at the leg hemline. Don´t cut the hem allowance, you need it as a hinge.
Now spread the slash for about 1 to 1,5cm at the back panel seam. Secure it with tape.
Measure how much the back panel seam got longer.
Draw a straight line across the center back piece from the single notch of the panel seam to the other single notch.
Cut this line and spread it the same amount you measured before. The panel seam line must have the seam length in the both pattern pieces.
Secure with tape.
Smooth the altered lines.
What do you want to achieve with this alteration? In long trousers this alteration is made to get rid of folds under the butt, so it seems pretty useless to me for boxer shorts.