according to the redbull site...
Front wheel
A 36-volt electric motor has been fitted to the hub of the front wheel to keep it spinning throughout the duration of the challenge. This spinning creates a crucial gyroscopic effect that will make the bike more stable for Dougie, and allow the bike to respond to steering inputs even with the wheel up in the air.
Dougie will control the power to the front wheel via a tension wire connected to his chest. Although the wheel will always be spinning, Dougie can vary the speed by changing his body position, allowing him to adapt to the undulations of the course and have more control over the bike.