An important component of upper body strength for inversions is the group of muscles that support the shoulder blade. As you move your arm into various positions to bear weight on it, the shoulder girdle (as the muscles that support the shoulder blade are called) must have the strength to support and hold each position.
The most important among these muscles is the serratus anterior. In most people, who are not used to bearing weight on their arms, the serratus tends to be weak. The preparation pose in the video helps one to prepare for headstand by working and strengthening these muscles.
