There's a groove running up and around the handle of the SlingsHOT mug, which both keeps the bag from falling into the cup, and also allows the user to pull the string back, slingshot-style, to squeeze water from the bag. That means no more squeezing with spoons or scalded fingers. Simple, but terribly effective.
Sadly, the cup is currently just a design concept by Samir Sufi—but it may yet make it into production. More here.