Well it's not some affliction caused by over doing on guitar, but it is one of these ...

... and it is claimed to aid sustain.
One arrived in the post this morning, and having tried it acoustically on both Pearly & JB I'm not so sure about the sustain having been improved, but I am very sure that the tonal response of both instruments is improved, and dead and live spots on the neck are evened out.
Notes ring out slightly louder too.
Well worth giving a go.