Many times while cutting a square in half diagonally, the square will slip at the last half inch of cutting. After experimenting, I discovered that I was not stabilizing the square under the ruler. Now I always place the tip of my index finger on top of the ruler, directly over the fabric triangle. This way the square cannot move because of the pressure on the ruler from my finger tip.
(Editor's note: Nancy is a quilt designer, teacher, lecturer and author. Her expertise is in rotary cutting and piecing quilts.)