Tied down spread eagle like that leaves our sub pretty helpless for whatever her Mistress has in store. The slapping and grabbing Mistress is doing serves a similar purpose to the spanking and paddling in the first couple of acts. It raises the endorphin release along with the anxiety level rather rapidly and either allows the sub to take more pain stimuli as the Mistress sees fit or provides a welcome relief and contrast should the Mistress take the opposite tack and go with pleasure stimulus. Either way, it serves an important function of heightening the senses in working the sub up for whatever comes next.
Caption © For the Love of A Submissive, 2012