One of the most common form problems in a squat (other than hip "ducking") is when the knees "pull together" when driving out of the hole. What SL is saying, is to make sure that you make an effort to try and spread apart your knees as up drive up - making the distance between them greater, or at least not less.
This is all no good though if you are doing a close stance BBer squat rather than a wide stance PL squat (which you should be doing).
Also, sit as far back as you can when in the hole. Do not let you knees get over your toes.
B-