The dome head sits magnetically within a rotating gyrowheel that powers the droid. The wheel features panels that contain many of the common astromech accessories (computer and ship interfacing devices), while the dome has the holoprojector and arc welder.
When powered down, the DD unit can sit flush with the floor.
One concept for how it gets back into a mobile position is using the spinning gyrowheel to propel it's dome up, much like a coin drop in reverse.
Because of the separate dome and wheel, DD units have the ability to become modular droids. Being able to swap in a heavier duty wheel for rough terrain, or a wheel with different accessories.