From Andrew Goldberg at Ifixit.org:

The board is also smart enough to not drive around without you. Riders need both feet firmly planted on the board or it won’t be going anywhere. Just how does the board know you’re properly mounted? Each foot pad has two infrared sensors—one at the toe, one at the heel. Stepping down on the pad pushes a peg between emitter and receiver. Only when all four sensors are blocked are you ready to roll...

...Those sensor switches live on the backs of the two gyro boards—one for each wheel. These boards are largely responsible for the “smart” part of smartboard. Each board is home to an Invensense MPU6050 6-axis gyroscope+accelerometer, and a GigaDeviceGD32F130 ARM Cortex-M3 32-bit microcontroller (thanks, Ken!). The ARM chips are responsible for reading the infrared switches, controlling the sweet underglow headlights and top-mounted indicator LEDs, and collating and sending data from the MPU6050 to the main board (more on that later)... (full article)