Learn head-to-knee pose (Janu Sirsasana) properly: fold from the hip, protect the bent knee and judge realistically what stretching delivers.
How far do you have to bend forward in head-to-knee pose for it to work? The honest answer: only as far as your back stays long. The name promises head on knee, but that is a distant goal, not a yardstick. Janu Sirsasana ("head-to-knee pose") is a seated forward bend over one straight leg while the other leg is bent and the sole of the foot rests on the inner thigh. It stretches the hamstrings and lower back and works well as an entry to seated forward bends.
What does research say about progress? I found no study on the pose itself, but on related postures: in a test with 150 healthy people aged 18 to 28, daily practice of a seated forward bend (Paschimottanasana) improved hamstring flexibility within four weeks, clearly more than the control group [1] (transfer, the leg is not bent here). In 31 young adults, a seven-week Hatha program brought about 4 degrees on average [2]. For you that means: it works, but with patience.
Sit in staff pose on the blanket. Bend one knee and place the sole of the foot on the inner thigh of the straight leg, not on the knee. The bent knee sinks to the side, not forced.
Turn your torso slightly toward the straight leg. Raise your arms, lift the spine and pull your toes toward you. This is the starting position of the forward bend.
Milestone: You sit upright for 30 seconds without the back rounding.
Fold forward from the hip, the breastbone moves toward the foot. If you cannot reach your toes, loop the strap around the foot and gently pull yourself in. Stop as soon as the back rounds. That is the most important signal in this pose.
Stay five to eight calm breaths. To release, rise with a long back. Switch sides and stay equally long on both.
Milestone to move on: You fold with a long back far enough to reach the foot (with or without a strap) and hold 1 minute per side. Then you are ready for the standing version.
Two spots are delicate: the back of the leg and the bent knee. In a review article on yoga injuries, hamstrings and knees are among the problem areas [6]. The knee is especially important because yoga practitioners in a review of nine observational studies had a higher risk of meniscus injuries than non-practitioners (odds ratio 1.72) [5]. So never force the knee into the posture. If you stretch deeply, part of the "progress" can turn out to be pure tolerance: four weeks of daily stretching raised tolerance by 8 degrees, while muscle length was unchanged [3]. Keep the stimulus moderate. This guide explains and motivates; it does not replace supervised training.
Use the strap as a loop around the foot. Instead of your hands, put a strap around the ball of the foot of the straight leg and hold the ends short. This is a practice trick: the back stays long, and you can tilt the hip deliberately. As a test: if you shorten the strap and your back still stays long, you have made real progress.
Only as far as the back stays long. The goal is tilting the pelvis, not head on knee. A strap helps. In studies of seated forward bends, hamstring flexibility improved within a few weeks [1][2].
On the inner thigh of the straight leg, not on the knee. The knee sinks to the side and is not forced. If it hangs in the air, support it with a block. This protects the joint [5].
These are variations from the Ashtanga tradition that position the bent leg or the heel differently. For getting started, the basic form described above is enough. Only when it works for 1 minute per side without a rounded back is it worth looking at the variations.
In the 150-person study, people practiced six days a week (4 minutes of forward bend per day) [1]. As a beginner, three sessions per week are enough. That is a practice recommendation.
In a study of 22 healthy people, heart rate and blood pressure changed measurably after 30 seconds of various postures, including Janu Sirsasana; diastolic pressure fell more after the left side than after a seated twist on the right side [4]. No therapeutic promise can be derived from that.
Pyramid Pose (Parsvottanasana): Standing Hamstring Stretch With a Long Back