If you keep leaning forward, your brain will keep putting your legs forward to keep you upright. Your brain knows you are out of balance, because fluid in your inner ear shifts, so it triggers you to put your leg forward and stop the fall. If you stand up and lean forward, so that you are out of balance, you probably won't fall on your face.
To understand how this system works, it helps to consider Kamen's model for the device - the human body. This balancing act is the most amazing thing about the Segway, and it is the key to its operation. To turn left or right, the rider turns the right handlebar forward or backward. To move forward or backward on the Segway, the rider just leans slightly forward or backward.
