"Wait a minute, these are two totally different technique," you are thinking. Think again. With this application of Putar Kepala your opponent's head is down (torso bent at the waist) and one of his arms is up in the air. The curved little fingers of mawashi uke are a clue that you are grasping your opponent in some fashion. In fact, you are grasping him behind the neck and by the elbow. Using the principle of the force couple (equal and opposing forces on a parallel path) you cause your opponent to twist and thereby lose his balance. Osae uke is representative of the force couple principle. Mawashi uke is a descriptive representation of what is going on (rotation).
Take a look at Lorenzo Bagnai putting Putar Kepala in action. Notice the use of forearm strikes to the neck, elbow strikes to the face, and knee kicks to the body to get the opponent into position. By advancing, retreating, or moving to one side or the other, he is able to direct his opponent in a variety of directions.