Overhead squats stabilize the core and engage the "11 line" muscles with their upper-body component.
"This workout involves walking with a weight above in one arm. Core activation resembles overhead squatting. I
Haines-Landram says "planks are both a popular and effective exercise for strengthening the core."
Rotations target the obliques, which border your "11 lines," giving them that distinctive look, using a dumbbell, medicine ball, or body weight.
Their static constriction of the ab muscles prevents them from moving.