From the description at YouTube:
These are two gigantic solid state musical Tesla Coils. A Tesla Coil is a special type of transformer invented by Nikola Tesla that is able to generating extremely large voltages using a phenomenon known as electrical resonance. Each coil in this video is capable of generating a 13 foot spark. This equates to about 500,000 volts of electricity.
The primary drive system for the coils consists of high power semiconductors arranged into an H-Bridge switching configuration. During a spark event, the coil is pulsed on for a few hundred millionths of a second. During this short time, thousands of amps circulate within the primary tank circuit and the energy is coupled into the secondary resonator through magnetism.
So what appears to be a continuous burst of sparks is actually a specific number of sparks generated per second. By modulating the number of sparks that emit from the coil each second, different tones can be produced by the coils.