They are called DAs because they have 2 separate actions running at the same time - while the big wheel goes around in big circles, the other action makes it go back and forth in little circles - the smaller the circles, the finer finish it will do. Quality sanders are available with different sized orbits.
A lot of DAs have a lock but it is intended to lock one action so you can change the sanding pad. If you run them locked, they tend to vibrate.