I regularly refer to this pose as one of my favourites and an excellent stretch for trampers. It’s designed to stretch the back of your legs including hamstrings and calves, as well as the outer muscles of the hip joint and depending on the variations, the inner thigh muscles as well. Stretching all of these muscles can sometimes help relieve back pain. And if you also take the time to make some small foot circles in both directions with your foot in the strap, you might find some relief for the finer muscles around your ankle joint.
Here’s brief (just under 2 minute) video that walks you through the basic points for the stretch.
Supta padangusthasana translates to reclining hand to big toe pose – because you’re lying down and in theory would be grabbing your big toe to create the stretch! In this more accessible version, we use a strap.