A gas turbine generator in simplest form harvests the power of expanding air to provide mechanical torque to spin a shaft.

Air is drawn into the low pressure compressor where it begins to build pressure by being forced into a smaller and smaller space. The high pressure compressor continues the process until the air has reached sufficient pressure. At this point, it enters the combustion chamber where fuel is burned. The resulting increase in temperature causes the air to expand rapidly. This expansion is the power source that drives the engine. The high pressure turbine uses the power from the expanding gas to spin the shaft connected to the high pressure compressor. The expanding air leaves the engine through the low pressure turbine. This power is transferred back to the front of the engine via a shaft to drive the low pressure compressor and the generator.

There are no physical connections between the low and high pressure sides of the turbine. Just like a fan slowly turning in the breeze on a summer's day, the force of the air turns the turbine blades.