The normal scenario begins when an image area becomes contaminated with dirt or dried ink. The printer sees the resulting print flaw and stops the press. The screen frame is lifted and the printer assaults the underside of the stencil with a solvent-soaked abrasive cloth. You're tearing my emulsion. A truly expert printer rarely rubs the print side of a stencil because he knows that print definition requires a sharp emulsion shoulder at all image edges.
Abrasion removes that necessary shoulder, and further abrasion scores channels in the emulsion.
Continued abuse actually removes the emulsion, leaving only bare mesh. Any further abrasion at this point results in that familiar shriek from both the screen and the press operator. While this combination of thin mesh and emulsion is remarkably tough, it will not withstand the violent physical abuse wrought upon it by unthinking printers. So the real key to avoid abusive cleaning is to prevent contamination of the stencil in the first place. The greatest cause of contamination is poor ink-management practices that lead to dried ink in the mesh.