This sensor was originally developed to position In-Furl-Booms correctly. To enable furling these booms have to be be leveled precisely within an 1° angle to the mast, otherwise asymmetric coiling will lock-up the sail. It has turned out that one could mark other usefully positions of the boom such as "home", "light wind" etc. also.
Once the boom is leveled into a position to be marked, moving the sensor along the boom until the LED flashes, indicates where the Marker should be fixed. This is done by turning the red LED housing clockwise. Moving the marker just a little bit away or altering the boom angle will switch off the LED.
Easy but useful, the customary batteries will last more than a year and can be changed without a tool. Several markers can be placed along the boom.