The full asana involves arm positioning as well but that hinders computer and other desk tasks. By encouraging the top knee across the body, this asana helps lengthen (release) the piriformis primarily. For full description, take note of caution points in the journal. https://www.yogajournal.com/poses/half-lord-of-the-fishes-pose
The twist in the hips and knee help to loosen these areas. Rushing a stiff leg to release will only cause injuries. Ensure position is comfortable; DO NOT push into discomfort, DO NOT sit too long to cause discomfort. Excessive practice can damage the superficial and deep peroneal (fibular) nerve!
Take note of safety and precautions.
Toe pointed position stretches the top of the foot and front of the shin. Toe flexed position stretches the arches and planter flexors. Together they soften the feet. Beware, the beginnings of this is often painful; keep it brief and do it often. If you have bad knees, use pillows to cushion and support.
This is my favourite to help with my running stride and gait. It opens up the front of the hips and top of the shins on one side while lengthening the buttocks and achilles on the other. With attention to the abdomen, it will condition the core. Pillow under the back knee to protect from sensitivity and sit on a block or stool to help with comfort. Many descriptions ask for the front knee to be at 90 degree but I feel if the knee is behind the toes and the center of gravity is behind the heals it should be safe if there are no sensitivities in the knee.
Don’t simply do this passively. The low back need to be engaged to arch just above the plevis and the tailbone should be lifted and abdomen tucked to lift the spine. Lifting of the tailbone tilts the pelvis forward on the top and back on the bottom where the hamstrings are attached hence lengthening the hamstring. Pulling the toes towards the body lengthen the gastrocnemius which attach above the knee over lapping the hamstring. This helps to lengthen behind the knee and separate adhesions between the gastrocnemius and the hamstring tendons.
Which you do really depends on how you angle yourself so why not do both? This stretches the hamstring and inner thigh depending on position. Pelvis and core should be engaged as per the staff pose.