From the product description:
The unit has a solid brass body and a glass cylinder and piston that are designed to minimize friction. The alcohol burner heats the glass tube, causing the air inside to expand and push the horizontal piston towards the chamber. This movement displaces some of the warm air into the metal chamber where it continues to expand, forcing the vertical piston upward. The pistons are connected to the flywheel with a system of levers and pulleys, causing the flywheel to spin up to 5,000 RPM and enabling the sequence to repeat until the flame is put out.
Here is the product page for The Desktop Stirling Engine.