Especially if you tend to focus too much on your knees, like I do.
Link: "Read this before you do another squat (then do several, after)"
A common cue used by trainers is "from the heel." I can't tell you the number of times Mat's told me to "sit" and "all the weight in the heel." My mind worries about my knees. But, I can tell when I'm doing it right because I have no knee pain. When I'm a little bit off, in my foot positioning, or because I'm getting sloppy in my form, then I sometimes feel the weird knee crunching or popping. It happens less and less frequently. When I was experiencing particularly problematic knees a few weeks ago, Mat would push against my knee as I squatted so that I could feel exactly the position I was supposed to be in.
I didn't understand why it worked, I just knew that it did. The article by Kevin Moore of Re-Embody explains in great anatomical detail why the focus should be less on the knees and more on the heel. Oh. Right. "Weight in the heel." Where have I heard that before?
But when the feet are positioned right, when the weight is properly in the heel, then the knees align themselves without needing to be manipulated. The tweaking and manipulation is much more important in the heel and ankle. Whether you have weight on your back, or it's a free-form squat. It's still all about the heels.