Do you mean a stud-finder? If so, then slowly wipe it across the wall until the red light or "beep" goes off. This tells you there is a nail or screw below the wall's surface.

Fits over the end of a slotted screwdriver bit, and then into a slotted screw. Prevents you from slipping out the end of the slot. You have to remove it to finish screwing in the screw. The ones with the sliding sleeves are much superior. Pretty useless piece of kit, IMHO, and not worth the time to figure out how it works. <g>

Rotzog is correct -- you take the small end of the plastic sleeve (screw finder) and place it over the screwdriver shank. If you look in the end of the plastic sleeve, you will see small metal tangs sticking into the opening. (These help hold it onto the screwdriver shank, and allow the shank to move downward through the screw finder as the screw is being installed.) You then place the head of a screw into the larger opening, and the "screw finder" will keep the bit from slipping off the screw as you start to drive it in. Why on earth they are called screw finders is beyond me -- they don't "find" anything. Their only purpose is to keep the screwdriver tip in the head of the screw as it's being installed. Hope this helps

Tool addict and Rotzog are spot on. The term "finder" is what throws people, today it would be called "holder", the wording goes back to the days of instrument makers and craftsmen who did every measurement and joint by hand with vernier gauges being the height of technology. Magical stuff.

1) "Slide on a quick change bit and screw The screw finder is for slotted screws. Using a power drill/driver with slotted screws in one of the most frustrating jobs for handymen. Unlike phillips head screws you must keep a right angle while driving and rely on your own coordination to keep the bit aligned with the slot. The screw finder slips around one of those long quick-change slotted bits and a flat head screw to take care of alignment." Source: http://wiki.answers.com/Q/How_does_Black_and_Decker_screw_finder_work 2) "Thoracic spine posterior transpedicular screw finder: The present invention relates to a thoracic spine posterior transpedicular screw finder that guides a drilling device quickly to the exact position for drilling purpose onto vertebra, secures the drilling process and prevents it from deviation in spinal instrumentation with the transpedicular fixation of spinal operation. " Source: http://www.freepatentsonline.com/20040073229.html 3) "Well, apparently you use it on the end of one of the driving bits, where it acts sort of like a collar to hold the screw in position on the end of the bit." Source: http://209.235.218.9/dc/dcboard.php?az=show_topic&forum=103&topic_id=4343&mesg_id=4343&page=3 4) "For driving long deck or wood screws. Screw Finder-outer sleeve slides over screw and keeps bit in screw recess. Screw Holder-holds screw in correct starting position without wobbling. For use with No. 10 and smaller screws. Refill for Bulk Display." Source: http://power-tools.hardwarestore.com/54-394-magnetic-bit-holder/screw-finder-and-driver-610554.aspx 5) a screw finder can also be some document you use to find the right screw for your needs:http://www.bobbydyer.com/projects/design/screw/screwfinder.html