It's been revealed how Forza 4 will use Kinect! When your car will have a breakdown, you'll be able to use it to: push it, change a wheel, hitchhike, wave your arms desperately to avoid being run over by other cars. When your car will run fine, Kinect will still have interesting uses: to pick your virtual nose and/or make rude gestures to other drivers.
Source: a high MS exec that wishes to remain anonymous.