Okay, I LOVE this adjustment and so do my students. (We do it with partners so everyone can do it.) BUT I am absolutely amazed at just how dorky I look in these photos. BUT since it is such a fabulous adjustment, I am going to show you the photos anyway! :-) (Disclaimer: My fabulous photographer did NOT take these photos! :-) )
This is really for Paschimottanasana, but it can also help students feel better in Dandasana as well. One of the challenges of Dandasana is using core muscles to get upright, not the superficial hip flexors. Many students grip the top of their thighs to keep the pose in integrity. This adjustment grounds the femurs so you can relax the superficial muscles and begin to learn how to use the deeper core muscles.
In this first photo, I have made a large loop with the strap and placed my hands in the strap. You want the strap to hit just outside the hips.
Then you take your legs over the strap and extend them. They will naturally press the thighs firmly down. Your student will say, "ahhhh...."