Attached is a video I made with Domingo and Kaz of Sugawara Dojo in Ridgefield, NJ. Here I demonstrate some applications of the two"blocks." Actually, the shuto uke in the video is not so much a block, as a check of my opponent's arm so that I may take his back.
The application for shuto mawashi uke is control. It is the gateway to many of the advanced applications found within karate self-defense technique (neck cranks, joint destruction, chokes, etc.)- no wonder that is appears so frequently in kata. Look closely, and you might see moves to break my opponent's floating ribs, hyperextend his elbow, gouge his eye while applying an elbow to his temple, knee his ribs, lock his neck, and unbalance him. That is quite a list of tasks. Luckily, we now recognize all of it as shuto mawashi uke.