Okay, two factors primarily influence which muscles will be stressed in a squat (I can’t remember which of King’s articles this is out of. . .):
Foot position: The closer the feet are together the more quad involvement. Of course this means the wider the stance the more glute/ham involvement. If you look at the way the WS boys squat you’ll see they take a huge base so . . . glute/ham style squats.
Body position: The more forward you lean (like a good morning) the more glute/ham involvement there will be. The more upright your torso stays the more quad involvement.
That having been said we can see that it’s not a one or the other situation. Your glutes, hams and quads will all be worked in all squats however.
Wide stance<------------->Narrow Stance
Big torso lean<---------->Little torso lean
So remember that it’s a continuum so if you’re aftera a glute ham workout then take a wide stance with a big torso lean. (the rason WS doesn’t lean the torso too much is because it increases the distance the bar has to travel)
IF you’re after a quad workout then take a narrow stance with an upright torso.
An easy way to keep that torso upright is to move the bar in front of you so try a front squat (bodybuilder or olympic)
Hope that offers some clarification.