Exactly, it is a version of "use it or lose it". Think of cave-dwelling fish who no longer use their eyes. They lose the capacity for sight. And once lost, it isn't coming back.
To give more detail, they were looking at an important protein which appears in many species. Since they know the protein for many modern species, and they know when different species branched off, they can reconstruct the likely form of the protein at each branching. Given modern computer modeling, they can figure out what each version of the protein will do. They demonstrated that it used to respond to 2 hormones. 40 million years later it responded to only 1 of those. They identified the responsible mutation and found that random mutations that happened after the first would cause it to respond to nothing if you undid the mutation.
In short, a significant mutation opens the door for further mutations that make the first irreversible.
To give more detail, they were looking at an important protein which appears in many species. Since they know the protein for many modern species, and they know when different species branched off, they can reconstruct the likely form of the protein at each branching. Given modern computer modeling, they can figure out what each version of the protein will do. They demonstrated that it used to respond to 2 hormones. 40 million years later it responded to only 1 of those. They identified the responsible mutation and found that random mutations that happened after the first would cause it to respond to nothing if you undid the mutation.
In short, a significant mutation opens the door for further mutations that make the first irreversible.