To begin in the back handspring you must start in the athletic stance, arms by your side, chin on a shelf or neutral, head looking forward, arms swing up to the ears before the body begins to extend.
As the arms reach the ears your body begins to extend one segment at a time beginning with arms and head, shoulders, upper back, middle of body with core engaged, then hips, knees and ankles. You end by pushing explosively through your toes. This extension ends in the body touching the ground with shoulders stretched.
It is called a tight arch or long arch. The rest of the body is tight and stretched. As the body passes through this position to a handstand, you snap your body down as a whole unit. As legs come down, arms and chest come up. They stay in a straight hollow position.