You can learn this pose in three stages, the last one looking like a tortoise with its limbs pulled in under its shell. It’s called Supta Kurmasana. Sit with legs extended, move them out beyond hip width, but not extended to sides. Bend knees, lifting them up, inhale, and on exhale extend torso forward, reaching arms under the knees, stretching out to sides. If you can, rest shoulders on floor. If this seems impossible, sit on a block, or pillow and see if you don’t have some success. Bring forehead to floor, or to a block, allowing legs to lengthen out straight, hopefully with knees over armpits. Try to keep entire foot pressed onto floor. If not, then press the heels down.
The second step is to reach the arms around the back, under the legs. bend the knees , lift chest up a bit, and try to clasp hands behind the back. Use a strap until it becomes possible.
Third stage is to interlock ankles and let head hang down as low as possible. You can come out of pose to change interlock of feet, or not.
Health Notes: This is a pose to assist in withdrawing the senses from the world, (Pratyahara) so it is a sacred pose to a yogi, as it holds the qualities of a stable mind. It is a good pose to practice for increasing equanimity, freeing the mind from anxiety.
Physically, it stretches spine, activates abdominal organs , and soothes the nervous system