Jazzy, you are probably thinking of the ToneRite for guitar. It is a small box that has an adjustable vibrator inside and rubber feet that clip on to the strings just above the bridge. You run it for at least 72 hours and it's supposed to "open up" the sound of the guitar. I've also heard of people doing this for many years by placing an acoustic guitar in front of a stereo speaker and then playing loud music for a few days. If you poke around the internet, you will find both empirical tests of the ToneRite unit where the guitar really sounds different, at least one scientific test that shows a difference in the frequency output of the guitar,before and after, as well as at least one scientific test that shows the ToneRite did nothing. I tried the ToneRite and it did seem to make a little difference on an acoustic guitar. It seems most people do this on acoustic instruments. Some people claim the ToneRite works on their electric guitars too. I think I tried it once on an electric and didn't notice much difference. I hope this helps.
Bill, tgo