A press release from the manufacturer (reported on by local blog Roosevelt Island 360) explains how it works:
The new "smart" parking spaces will be outfitted with Streetline's patented parking system which includes ultra-low power sensors that communicate with one another to deliver valuable real-time information, such as how long a car is parked and when a car enters and leaves a parking space. The initial system also lays the foundation for smart parking meters allowing for easier payments and better pricing."
Right now, only 29 spaces have the sensors as the city performs a test run. Once the program spreads, the city is developing an iPhone App that will direct users to empty parking spaces.
Thanks to Noah Kazis