The idea of the knee not moving forward in the descent of a squat or lunge is really a weight distribution question that relates to the foot and ankle more than to the knee. In reality, the knee is only moving forward based on what goes on at the ankle and at the foot. In effect when we tell someone not to let the knees move forward when descending, we are really trying to correct a weight distribution problem, not a knee movement problem.
The question really is whether the weight should be on the balls of feet versus the heels when squatting. In order to control knee and ankle motion we cue our trainees or athletes to sit back and keep the weight on the heels when squatting. Getting the weight properly distributed causes the knees and ankles to move in sync and results in a great improvement in squatting. Telling an athlete or client to not let the knees go forward is the wrong cue to correct a problem that emanates from the foot.
Next time you see someone squatting with the a "knees first" approach, look at their feet and ankles. I will almost guarantee you that their weight will be on the ball of the foot and that they will move initially from the ankles. Moving initially from the knees versus the ankles actually results in a "sitting back" action that will greatly spare the knees.
The truth is that the action should initiate from the hips and knees and not from the ankles. Pushing the knees forward is clearly a flaw that should be corrected but, the reasoning has been all wrong. In any deep squat the knee will be over the forefoot. The question is not where does the knee go as much as where is the weight distributed and what joint moves first.
People get too focused on the knee position. They should focus on hip position and weight distribution. To teach squatting, forget about what the knees are doing and focus on weight distribution at the foot and action at the hips. Remember, the knee is just a hinge stuck between the ankle and the hip. It doesn't do much on it's own.