First and foremost. I apologize that the audio is bit low!
This mobility drill is a great way to improve quad and hip movement. You will be mostly stretching the quad in the bulgarian split squat position but when you tilt to the opposite side you will be lengthening and mobilizing the hip flexor.
This exercise is a great way to help those with knee, hip, and lower back pain as the hip flexors are a major contributor to their dysfunction! Do this a few times on each leg then repeat 2-3 times.