It is a common drill or cue for maintaining Parallel shins. Think of the shins moving like the windshield wipers of a car: they stay parallel and move together in an arc.
Pay attention to the shins throughout the run below:
The idea is to completely avoid any pivoting or other inefficient movements and focus on just switching the edges from one side to the other, carving shallow turns while focusing on maintaining the shins at the same angle (i.e. the skis at the same angle).