DirectVR rotation means that rotational tracking is directly translated into a game’s camera rotation. The main advantage over vorpX’s default head tracking is that it never has to be calibrated. In older games it also improves latency.
In most newer games, e.g. recent Battlefield titles ;), the default mouse based head tracking is almost as good as DirectVR rotation would be.
You can turn your head in any game that supports looking around with the mouse. Additionally vorpX can emulate a gamepad stick with head tracking (useful e.g. for racing games without mouse look), although that is rather laggy.