Ben Youngs try vs Gloucester

When I saw Ben Youngs' try yesterday, I immediately thought double movement. The argument is that he wasn't held by the tackler. But the laws state that the tackler must release the tackled player once the tackle is complete. The flip side to that coin is the tackled player must release the ball before continuing i.e. release and pick up again before scampering under the posts. At no point after being tackled did Youngs release the ball before crawling over (without the assistance of momentum). Can someone with a better understanding of the laws explain this one?