On page 52 of this book you will find this reference. It has been a while since I polished a telescope mirror, but I believe the condition he is talking about is when your polishing 'pitch lap' dries out and begins to squeal as you continue rubbing it against the mirror. The mirror will eventually take on a polished sheen, however although this is desirable for commercial plate glass mirrors, it is a useless condition for telescope mirrors. In the latter case you are trying to use the rouge to fine-figure the shape of the mirror to a parabolic curve. Once the rouge dries out, you loose most of your control over the generation and fine-figuring of this process at the fraction of a wavelength tolerances you need.