My PT introduced to a very slight variation on that stretch.
It is basically the same, but instead of pulling your knee to your chest, keep your femur at a 90 degree angle to the floor. Keep it there with a towel wrapped around it that you can pull on to keep it upright. Now do leg extensions while in that position. On a good day I can get my knee angle open to about 135 degrees... yeah, I'm inflexible.
I have also found that I prefer doing it without any lordotic support. That way it forces me to consciously think about maintaining it myself. When I was using my Abmat, I was relaxing too much and end up letting my pelvis rotate.