Once built, the Solenoid engine twin should tick over easily. It will take a nudge to get it started
To increase the speed, hold the button in the same direction that the flywheels are going, the speed should increase.
To decrease the speed, hold the other button down, If you slow the engine right down the engine will (once it has slowed down enough) reverse direction. Continuing to hold the button down will speed the engine up in the new direction.