How On,Off and Reserve works in bike fuel tanks

Generally, a motorbike has fuel in the tanks at three levels:

1. Unusable Fuel (Red Part)
2. Reserve Fuel (Blue Part)
3. Normal Fuel (White Part)

Now a bike's reserve switch has 3 options, Fuel off, Fuel On, Reserve (Considering the old prime technology not being used anymore).

Switches:

• Fuel On: The white part/ normal fuel is accessible
• Fuel Off: No fuel is accessible
• Reserve: The white as well as the blue part/ reserve fuel is accessible

NOTE: The red part's fuel is never accessible. Also, not all bikes have the this feature.

How Reserve works: 

As per above images when knob is switched to On position fuel access only through On pipe and when fuel is below On Pipe fuel stops flow so that bike stops with a jerk and hence we need to switch knob to reserve position so that we can access remaining amount of fuel.