 In down position the propeller is stopped and the instrument is lowered, after which reaching the desired depth a weight called a messenger is dropped to move the lever into the middle position which allows the propeller to turn freely.When the measurement has been taken another weight is dropped to push the level to its highest position at which the propeller is again stopped.The propeller revolutions are counted via a simple mechanism that gears down the revolutions and counts them on an indicator dial.