Happy baby pose (ananda balasana) is one of my favourite poses for relaxing the lower back, working deeply into the hips, and quietening the breath. It may not be one of the ‘classic’ poses found in the ancient texts, but there are many benefits of it.
Because you are lying on your back, the pose immediately feels more relaxing than holding an equivalent position on your feet (which would be a low squat pose). The back is fully supported on the floor so your back muscles can relax, and you can manoeuvre the lower back so that it feels lengthened, which is important if you spend a lot of your time sitting and compressing the lumber spine. With a deep fold at the hip crease you will really feel some work in your hips, similar to a deep squat or paschimottanasana.
To come into the pose, lie on the floor and bring your knees over your chest. Edge the knees and thighs to each side, so that the knees are moving towards the armpits. Hold the outside edge of each foot with a hand and raise the foot so that the shin is vertical and the sole of the foot faces the ceiling.
If you find it difficult to be in deep forward folds your thighs may not come down very far, and, if you have shorter arms as well, you may find you can’t quite reach the feet with your hands. If this is the case, hold the shins or back of the thighs, or use a strap around the ball of the foot. You can do this with both feet at once, or one side at a time, as illustrated in this article.
After some breaths in the pose, if you want to move into a stretch for the hamstrings, it is easy to straighten the legs diagonally away from the body t an angle, so the feet come over the shoulders.
As a counterpose, it is nice to follow this with supta virasana, or a prone backend such as salabasana, both of which would provide a counter stretch to the hip flexors.
If you like to read about mythology associated with yoga poses, this article has a modern mythology story (along with a more detailed description of the pose) of happy baby pose.