The video above is a classic quadriceps stretch and well as a standing psoas stretch . The quadriceps is a group of four muscles that connect below the knee and then three of the four muscle connect on the femur or leg bone and one (rectus femoris) attaches to the pelvis. This connection at the pelvis is responsible for a great deal of hip problems. In yoga I refer to this problem as a seizing of the hip flexor where the very top of this muscle engages and grips like a steel band that won’t relax. You know what I am talking about if you fall into that category.
The psoas attaches at the base of the rib cage, along the lumbar spine and then crosses the rim of the pelvis to move backwards to connect to the back half of the inner thigh along with the iliacus. The standing psoas stretch is only available if the alignment of the shape is correct. It is easy to misalign the leg and pelvis and take any standing psoas stretch out of the equation. The quadriceps stretch is much easier to find.
The key to finding either the quadriceps stretch or the standing psoas stretch is to set the shape up correctly. The most important aspect of this alignment is to have the trunk facing forward the entire time. Both sides of the pelvis should be pointing straight forward  in order for the leg to move backwards in a way to access the psoas.
The main culprit in pulling us out of either the standing psoas stretch or the quadriceps stretch is the IT Band, or iliotibial tract, a strong band of connective tissue that runs down the outside of the leg with a small muscle, the tensor fascia latae, embedded within it. Tightness in this muscle, which tends to be very tight in anyone with poor posture, will pull the knee out to the side as soon as you start pulling the leg back.
This is why focusing on the inner thigh to pull the leg into the correct position in opposition to the outer thigh is key to achieving the proper stretch. The inner thigh can set the psoas back at its base and then toning the lower abdomen can pull the psoas up at the top. This alignment brings you into the standing psoas stretch, and the quadriceps stretch is icing on the cake.