With the extreme range split squat or lunge, you're going elevate both the front and back foot. This will allow you to use a longer range of motion than the regular split squat.
The knee of the back leg should not touch the floor, but it should end up lower than the front foot in the bottom position.
Since many people struggle with squats because of flexibility or mobility issues, performing this split squat variation first will loosen up the hip flexors and allow for a deeper, safer squat. Just hold the bottom position of the extreme range split squat for as long as you can, alternating legs, before doing squats.