This subroutine alternates vinyasas of two postures, apanasana, pelvic floor poses and pavanamuktasana, wind relieving posture.
In all the anapanasana vinyasas the head stays on the mat, the hip lifts to bring the knee or knees to the chest.
In all the pavamuktasana vinyasas the head lifts off the mat to bring with the chin, nose or forehead to the knee or knees.
It's acceptable to raise the head when catching the knee in anapanasana but once caught lower the head, and tighten the grip around the leg or legs and press the thigh(s) against the lower abdomen.
In the pavamuktasana vinyasas be careful not to strain when bringing your forehead to your knee, stretch up through the length of your spine and drop your shoulder blades down your back.
Engaging uddiyana at the end of the exhalation may also help to allow the forehead to reach the knee.
Ramaswami includes anapanasana along with urdwa-parasarita-pada-hastasana ("U" formation) and dwipadapitam (desk pose) as important sarvangasana (shoulder stand ) preparation.
I also find anapanasana and urdwa-parasarita-pada-hastasana ("U" formation) to be excellent counter postures to intense back bending along with yesterday's tatakamudra ( pond gesture).