As the name implies these are stop valves that use a ball to start or stop the flow of liquid. 
When the handle is operated to open the valve, the ball rotates to a point where the hole through the ball is in line with the valve body inlet and outlet. 
To shut it requires a 90 degree rotation of the hand wheel or lever, the ball rotates so that the hole is perpendicular to the flow openings of the valve body.

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