Rear mech cable tension

Hi
I'm going to struggle describing this but here goes anyway...
A while ago I noticed a device that fits behind the rear mech and cable adjuster that looks like some kind of cable guide with a jockey wheel that (I think) is either an alternative to the adjuster bolt or works to enhance the fine tuning of the tension somehow?
I don't know what they are and I cna't seem to find them anywhere...
excuse the crude illustration but they look a bit like this...

Anyone any ideas what they do?
I'm going to struggle describing this but here goes anyway...
A while ago I noticed a device that fits behind the rear mech and cable adjuster that looks like some kind of cable guide with a jockey wheel that (I think) is either an alternative to the adjuster bolt or works to enhance the fine tuning of the tension somehow?
I don't know what they are and I cna't seem to find them anywhere...
excuse the crude illustration but they look a bit like this...

Anyone any ideas what they do?
get on your bikes and ride!
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Posts
to remove the big loop of cable on a shimano mech.
one of these
Parktools :?:SheldonBrown
(my illustration wasn't too off then...)
"they also can help with niggling indexing problems...." I wonder... (trails off in thought...)
...
...
...
but then again one of the old XTR mechs had something similar built in!
Parktools :?:SheldonBrown
I see that image is from Wiggle... but I can't seem to find it on their stockllist?
but now discontinued.
use a V brake noodle.
Parktools :?:SheldonBrown
that thought had crossed my mind...
unfortunatly they wont. as avid do not make them any more.
Parktools :?:SheldonBrown
Parktools :?:SheldonBrown
maybe when I get a job I can think about that kind of upgrade... but as it is... cheap mods for me...