Regulars to my classes or workshops will be very familiar with this: the supta padangustasana or hand to big toe series. It helps you access the muscles on the back, inside and outside of your leg/hip.
To practise this at home you don't need a proper yoga strap. You can use a dressing gown (or normal clothes) belt, a scarf, towel or pretty much anything you have to hand. One really common issue with this posture is that when people (including me) take their top leg wide (ie top right photo) they find it really difficult to keep the bottom one anchored down. If the bottom leg lifts in this position then the action of taking the top leg wide is going to come less from motion in that leg, and more from the pelvis tipping to one side. So you won't be getting much of a stretch along the inside of your top leg because your pelvis is just tilting to the side.
There's an easy way to stop that happening. Say you're starting with your right leg: lay on the floor with both legs straight. Take your hand to your right hip and find the bony part at the side, at the widest part of your hip (anatomy fans: this is your greater trochanter). Put a yoga block on the floor next to you, snugly up against that bony part on the right side of your hip. Probably most comfortable will be one of the flatter blocks, but if not a brick block will work too. And if you don't have blocks try a firm cushion, or a rolled-up towel, or a book (whatever you have nearby!). Leaving that support where it is, bring your right knee into your chest, place your strap around the ball (widest part just below your toes) of your right foot and take that leg up to the ceiling. Then take that top leg out to the right. Notice how the support next to your right hip helps stop the weight of that leg pulling your pelvis off centre. And if your pelvis can stay square, the motion will be coming more from your leg and you may find you get a more effective stretch on your inner thigh. Most people also find that being able to relax into the support by their hip means they can hold the posture a little longer too.
There's also a simple way to deepen the stretch in the bottom right photo. Say you have the strap in your left hand and are taking your right leg across your body (as I am in the photo). Take your right thumb into your right hip crease, so below the bony part at the front of your hip and just where the top of your leg joins your torso. Your thumb is right at the very top of your leg and your fingers wrap around towards your bum. Use that hand to move your hip away from you. And rather than just pushing with your hand, instead think roll your outer hip down and away from your face. The hand holding the strap makes sure that your foot doesn't move away from your face at the same time. It's a reasonably strong movement with your right hand (within reason, obviously!). And remember: don't just push with that hand; roll your outer hip down and away from you.
See how these adjustments work for you and send us a message if you have any questions!