High bar & Low Bar squat differences

First, let's talk about the differences:

One has a very upright torso with a lot of forward knee travel (high bar) and the other has a bent-over torso with a very particular backward shift of the hips (low bar). 

The difference between the two comes down to where you place the bar on your back. The higher bar placement rests the bar on top of the shoulders and traps, while the lower position rests the bar on the rear delt across the lower traps. 

The main difference between the two is the muscle groups used. With a high bar you primarily have to use your quadriceps to straighten out the knees and force your body and the weight of the bar upwards. This is great if you have very strong quads and short femurs (relative to the rest of your body) as it creates a very short movement that can be very efficient if you have the correct proportions. Sadly, most of us don’t have very short femurs and ridiculously strong quads. 

Now, let's talk about how to work with them properly:

In order to have a good squat (high or low) the bar must be placed right above the center of your foot if you were to draw a line from the bar straight down. Maintaining the bar over the midfoot throughout the squat will allow for the most efficient bar path possible- a straight line (look at the picture)

In the high bar squat, maintaining the bar over the midfoot is relatively easy as you start in that position naturally with an upright torso. And on the low bar squat, you must also push the knees forward but to a much lesser degree, as you also shoot the hips back allowing the bar to descend over the midfoot. The ascend should look relatively similar to the descend for both high bar and low bar. 

But also try to mix them up because both high bar and low bar have their place on a powerlifting program. The high bar position is very good at building quadriceps strength and can help manage fatigue on a low bar squatter. A low bar may help increase posterior chain development (glutes, hamstrings, and back) and express top-end strength. Give both a try on different squat days throughout the week with appropriate load and periodization to reap the maximum benefit. 

I hope it helps you