Rotating paddle type level switches are basically a spring loaded, eccentricaly driven rotating vanes or blades which rotate at a very slow rotation of one revolution per minute, using an ac synchronous motor.
When no material is present, rotating vanes, (or blades or paddles) are free to move and motor is held by a spring in its normal position.
When material reaches the rotating vanes, it tries to stop the rotating of vanes, causing a reverse rotation of motor and stress on the spring which is holding the motor.
When motor rotates to a degree where limit switch is present, the power to motor is turned-off and output is generated.
As soon as material level falls, the vanes become free to rotate once again, causing the motor to turn back to its normal position due to stress on the spring holding the eccentric-drive, synchronous motor.