ElectroMagnetic Sensor, are based on an electromagnetic waves with low energy.
When reverse gear is selected the small control unit generates an electromagnetic field and transfers this to the transceiver adhesive strip that is placed on the inner side of and run the length of your bumper.
But in my opinion, it gives false alarm from time to time. Easily to get interfered by around environment such as car DVD, other electrical equipment. the ultrasonic parking sensor technique is more mature than the electromagnetic parking sensor.
I took some parking sensors from http://www.carsda.com , they have some fashionable designs, over 9 months, it is still ok.