Broke in guts are like a broke in pair of boots, everything fits better when they been broke in. The reed is worn and so are the gut pieces, only a minute change, but the same with a pair of boots, after you have worn them a couple of times they just fit better.
Like was said, hook up a vacuum exhaust and shoot some air in to the call, they will become worn in or broke in. I have heard of people making call bodies just for that purpose, where a vacuum exhaust hose fits the end of the call barrel. I have never seen one set up, but curiosity killed the cat so I tried it, it made noise. So the theory does work. But take it all with a grain of salt, I am not much of a goose call maker. Just not much need for them in arid west Texas.
Marvin