I recently bought one from Jim Scimonetti - a silk-handerchief kinda thing with a weighted end on the string. I rarely have used such things but at the time I bought, I was playing a Yanagisawa S901 soprano that constantly collected moisture in the upper octave vent. I thought this might solve the problem. It didn't.
So now I keep it with my soprano of choice (any of seven that I own) and just today, used it at the end of a gig on my S992.
Frankly, I'm not convinced it has any use other than to give the impression I know what I'm doing. DAVE