That dismount is really called the Ferris Wheel dismount. I don’t know if anyone uses the term for the somersaults by themselves like in that tutorial, but it is the common name for the dismount.
If you are getting the string tangled, you might be swinging the yoyo around the outside of your throwhand arm. When you do the roll, you want to swing the yoyo between your throwhand arm and your body. You can see it pretty well in the side view of the video GregP posted. If you swing the yoyo outside that arm instead of to the inside, it will get tangled.