When we look at the squat we are going to be discussing the difference between narrow and sumo squats, and high bar vs low bar. I will use two pictures to describe what happens when you do not pass 90 degrees It is very important to past 90 degrees
When going past 90 you are going to be putting full stress on the Quads, in which will build your quads up better then half squats. This will also help you to make depth for a power lifting meet. This is where most people bomb out in meets, because most people do not have a way to train to depth. I use those "boxes" to check depth, so when it comes to meet day I will know when I pass by the feeling I get in the hips and knees. You will find it might be harder to make depth on sumo then narrow.
When narrow squatting, you will have a lot of stress on the quads over sumo squatting. When narrow squatting you have to remember you have to go further down in your stance. For Sumo squatting you will need to be flexible at the hips and groin area, so you will need to strech a lot more. when sumo squatting there will be less presser on your knees compared to narrow. When talking about sumo, you will see a lot of people use low bar.
With low bar I found it helps to get the depth when loaded with enough weight on the bar. You will have to be carful when using low bar becuase there is a higher chance of the bar slipping off your back. With High bar squatting you need to find the sweet spot on your delt area. If the bar is to high on the neck, then watch out the next morning because you will have a very stiff neck.This is due part, because most people look up in there squat. When the bar is to high it sits on the traps, which is bad when looking up towards the sky.