Here are 3 different types of Hatfield split squats. Each one targets the hips and knees in just a slightly different way.

1. Standard Hatfield split squat (hip emphasis). First up is a standard Hatfield split squat, done off a deficit. It's a crappy camera angle unfortunately, but the deficit ensures a large range of motion through the hips, where you reach a position of deep hip flexion on that front working leg. There is some forward knee travel, but the primary goal is to move the hip joint straight up and down and really emphasize the action taking place at the hip rather than the knee.

2. Hatfield split squat (knee emphasis). Next up is the same exercise but this time emphasizing the knee instead of the hip. So here you can see I'm allowing my knee to travel way forward over my toes (almost like a Hatfield variation of the ATG split squat popularized by @ ) and even allowing my heel to come up off the ground to help me emphasize this aspect and really hone in on stretching out that VMO with the deep knee bend.

3. Deficit reverse lunge. Lastly, we have a deficit reverse lunge. So here instead of performing a stationary split squat I pull the leg all the way forward and then lunge back again. And that action of actually bringing the leg back to center provides a different stimulus to the hip musculature that you don't get from the static split squats.

Deficits allow for greater range of motion at the hips and knees. They should be graded to the individual. The ones demonstrated in this short video are rather large.

Remember, with these Hatfield variations the goal is not to PULL with the arms, but rather to gain stability that allows you to overload these unilateral patterns in a way that you cannot do with traditional unilateral exercises. As far as single leg work goes I find these variations to be the most fun, most challenging, and most productive of all. Good luck!