Swanny discusses his grip as well as how he produces variation. He does get good variation by adjusting the ball in his hand slightly so he is not gripping every ball exactly the same. When the ball lands on the leather it tends to skid on towards the batsman. If it hits the seam it will grip and turn more. He does also have the arm ball which comes out more seam up.
It is important to understand that not all off spinners grip the ball the same way. This is just the way Swann does it. He is aggressive through the crease and has a strong, flexible wrist which allows him to impart serious revs on the ball.