This is a very deep Forward lunge that works the end of your range of motion in Hip extension and Knee flexion .
When reaching the bottom position of the lunge, the front leg's hamstrings should come into contact with your calf (aim for full Knee flexion ).
Aim to keep your back leg as extended as possible; your knee is going to bend a little and that is ok, but aim to keep it straight, to get a deeper stretch in your hip flexors.
It is acceptable for your front heel to come slightly off the ground.