Segway PT x2 Reference Manual page 26

When you lean, your center of gravity moves beyond the contact
patch (the place where the tire contacts the ground) causing the
Segway PT to tilt. The Segway PT's systems sense this change,
powering the Wheels to keep them underneath you. See Figure 3.
Figure 3. How the Seqway PT Works - Leaning Forward and Back
When you pivot the LeanSteer Frame left, the Segway PT turns left.
When you pivot the LeanSteer Frame right, the Segway PT turns
right. To turn the Segway PT, point the LeanSteer Frame in the
direction you want to go, keeping the Handlebar in line with your
body. When the LeanSteer Frame is centered, the Segway PT travels
straight ahead. See Figure 4.
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