couper les coins

CONT: For this, one must, in each corner or, in other words, at the end of each straight line, make the shoulders enter the corner while keeping the head placed to the inside; and, at the same time that one turns the shoulders onto the other line, one must make the haunches, in their turn, pass in the same place on the corner as did the shoulders. ... This is what I call taking the corners ...
CONT: When the horse has started to obey in both directions at the shoulder-in lesson, one will teach him to work the corners well.