Sharping levers are used on folk harps to facilitate key changes. Installing a lever over a string allows you to raise the pitch of that string one-half step by engaging the cam handle against the string. Thus an F-string can be raised to F# by a simple flick of the lever. Similarly, a B-string may be tuned to Bb so that the lever will raise it to a B-natural and release it back to a B-flat, as needed
Most folk harp players set the key signature (sharps or flats) on the harp before starting each piece of music. For the key of G, you would engage the levers on all the F strings to produce the F# needed for that key (making sure all other notes on the harp are natural). If the following piece were then to be played in the key of F, you would then release the levers on all the F strings to produce F-natural, and also release all the B-string levers to produce Bb.
You may install a lever over every string on the harp, but if you think you may never use all of them, it would be more cost-effective to select which keys you think you are most likely to use, and then install only the levers necessary for those keys:
KEY OF G: requires F#
KEY OF D: requires F# and C#
KEY OF A: requires F# and C# and G#
KEY OF F: requires Bb
KEY OF Bb: requires Bb and Eb
KEY OF Eb: requires Bb and Eb and Ab
I picked up an old Triplett harp with no levers in fairly good shape, but it had no levers, so I wanted to try to make the instrument a bit more versatile. I also do not have a great attachment to the harp as it is likely in its last decade of life. So I tried out these levers. They are one-size-fits-all, so there are no worries about having to buy specific ones. They were surprisingly simple to install. Now I can play in 3 keys on this sweet little instrument!