How are brainwaves "read" and converted in to energy?

Brain waves are tiny electrical waves released when a neuron depolarizes, or "fires". Lots of these little waves come together to form larger more complex waves. These waves are detected by the sensors on a NeuroSky headset which then creates and electrical signal within the device that corresponds to the wave patterns detected.

This can be thought of as a microphone recording the sound of an orchestra. Each instrument creates it's own sound waves. These waves then come together to form larger more complex waves. These more complex waves are detected by a microphone which then creates electrical signal within the microphone that corresponds to the wave patterns detected.