Out of the several different arm/hand positions for Head Stand, there are two that are the most frequently practiced: Sirsasana I – where your forearms form part of the foundation, and Sirsasana II – or Tripod Headstand where your hands are on the floor with your arms bent at 90 degrees.
People often have a preference, one that they like better than the other. Often this has to do with their ability to perform the pose. If your shoulders are tight, Sirsasana I is harder. If your arms are weak, Sirsasana II is harder. While preference is fine, there are reasons to work on both variations. Each headstand requires different competencies and each headstand prepares the body for different further variations.
Sirsasana I
Typically the first headstand taught is Sirsasana I. Because it is a closed chain movement, where the hands are brought together and the fingers interlaced, it makes a closed circuit of the forearms, it is the most stable of the two and it strengthens the shoulders for other inversions and arm balances. But, it requires shoulder flexibility, specifically the ability to raise the arms overhead. For some students, this is difficult. We hold a lot of tension in our shoulders and if we have a slightly rounded upper back, this inhibits the range of motion of the upper arm. Sometimes students exhibit what is referred to as deceptive flexibility. An example of that is when a student raises their arm overhead and their back arches. This makes it look and feel as if there is flexibility in the shoulder joint, but there is really compensation in the back. This range of motion is an important aspect of poses where the arms are in the overhead position such as: Down Dog, Warrior I, Chair Pose, Wheel, Handstand, Forearm Balance and Cow Face Arms. If you have shoulder tightness, these poses may present a challenge for you.
Sirsasana II
Being an open chain movement, where the elbows can move in and out, this pose is less stable than Sirsasana I. It requires less flexibility because the arms are straight out from the shoulders, rather than overhead, but you need more strength for Tripod Headstand than you do for Sirsasana I. If your shoulders and upper arms are weak, this position will feel wobbly. This upper body strength is relevant to other arm balance poses where the arms are also bent 90 degrees: Low Push Up, Crow, Side Crow, Peacock, Swan, Dragonfly and those crazy arm balances named after sages such as the Koundinyasa variations, the Galavasana variations.
Sequencing
Although the two Head Stands are related, notice there was no overlap in the poses I associated with each headstand. When I am sequencing a class, I think about logical progression of postures. Practicing yoga is about a progression of skills rather than a performance of asana. While you may be proficient in one version of Headstand, it is a good idea to practice both variations to increase your repertoire, strength and range of motion.