There are some extremely common and glaring errors that many people make. I will outline three different types of form that I frequently observe, and will discuss what technique I think is the most conducive to achieving the best results. While of course it always depends on an individual basis, there are some glaring mistakes that need to be addressed.
#1: How Many People Perform Split Squats - (upper video)
When a split squat is being executed to perfection, close to 100% of your weight will be on your forward/working leg. However, it is extremely noticeable that my body is traveling in a horizontal path, and that my weight is being transferred to my back/non-working leg. My forward leg is doing very little work, which renders the exercise less ineffective. In addition to this, the knee of my working leg is in a vulnerable position. Due to trying to ''cheat,'' the emphasis is on my back leg instead of the forward/working leg, and as a result, my quad, glute, and hamstring are providing this leg with very little support. Many people make this mistake.
#2: How Some People Perform Split Squats - FORM IS SO-SO But Could Be Better (middle video)
While my body isn't traveling in as much of a horizontal path and my weight isn't shifting to my back/non-working leg to the same extent as in example #1, due to my hip positioning (my hips are positioned way too far back and towards my back leg), and the positioning of my knee of my forward/working leg (above the ankle instead of closer to my forefoot) there is about a 50/50 weight distribution between my front and back leg. This is not conducive to achieving optimal results, as with a split squat (and lunges), the forward/working leg should be doing as close to 100% of the work as possible. That being said, people who have longer femurs do tend to feel better in this stance and it often works better for them. See what works best for YOU.
#3: How Very Few People Perform Split Squats - (bottom video)
This is how I like to perform my split squat variations. Notice how I exaggerate and squeeze my glutes and push my hips forward so the knee of my forward/working leg is closer to the toes (versus being above the ankle like in #2). I do this before the start of each set. Positioning my hips so they are closer to my front foot loads the quad and glute of the forward/working leg and makes the exercise much more effective and challenging. I've worked with many people who thought they were performing split squats properly, yet were barely able to do them when I forced them to adopt this stance as their quads and glutes were not used to working this hard, and they had become accustomed to ''cheating'' and transferring their weight to their back/non-working
It's important to note that while my knee is close to my toes, I have a tripod base and my weight is on the mid/back of my forward foot and all of my toes are down. If you have longer femurs, knee issues, or poor ankle mobility, you might prefer stance #2. However, if your form remains solid, you will likely find that #3 is your best choice. Like everything, figure out what works best for you.
A few more key points. It is also important that your pelvis remains close to level. I like to use the water glass analogy. If your pelvis is level, no water should spill out the front, back, or side of the glass. If you want to make the exercise more quad dominant, keep your torso more upright. Conversely, if you want to target your posterior chain to a larger degree, you can lean forward a little bit more (but not too much, and keep your spine in neutralish alignment), and adopt a slightly greater stance between your front and back leg. Like everything, figure out what works best for you.