Definition - What does Headstand Split mean?
Headstand split is a variation of headstand, where the yogi opens the legs along the midline into a split. Also known as Utthita Pada Sirsasana, this intermediate to advanced posture allows the yogi to play with balance while also improving mobility in the hip flexors and strength in the core. Headstand split is entered from headstand by engaging the core and using muscular tension to bring the legs into a split.
Yogapedia explains Headstand Split
While this posture requires balance to maintain, the key to headstand split is a strong core, and flexibility in the hip flexors. As the legs open along the midline, the core stabilizes the upper body and supports the legs and the movement of the body. The forearms are active here as well to keep the base stable and to alleviate any pressure in the neck.
Opening the legs into the split while in an inversion requires the use of muscular tension, as gravity does not play a major role here, and the natural tendency will be to turn the hips out. By engaging the inner thighs, the legs stay internally rotated and the hips stay in one line. When first attempting this posture, yogis might not be able to bring the legs horizontal, even if they have the available flexibility, however, overtime, the strength needed in the hip flexors will be developed.