What'a a rotostub tuner do?
 

I came across this item:
http://cgi.ebay.com/06-02474-Rodostu...em180020464744
while looking for double-slug tuners (anyway have one they want to sell?
:-) ) and it's not at all clear to me what it does. Does it just provide a
few degrees of "line stretching" as you rotate it to provide fine tuning of
a match? Or something else?

I did some Googling by Teledyne is notoroius difficult to get useful
information from!

Thanks,
---Joel