The trick is simple and brilliant. Benjamin Long, Sue Ann Seah, Tom Carter and Sriram Subramanian, researchers from the Bristol University pioneered into 3D surface rendering. Ultrasound waves from multiple sources are combined in a way that the individual waves interact with each other, either amplifying or cancelling different regions. They are precisely calculated so that the regions that are amplified generate eventually 3D objects in thin air!
The method uses ultrasound, which is focused onto hands above the device and that can be felt. By focusing complex patterns of ultrasound, the air disturbances can be felt as floating 3D shapes... The system generates an invisible 3D shape that can be added to 3D displays to create something that can be seen and felt.