That would be an (edit: white) pawn moving down the g file towards the (edit: black) pieces and I have no idea why someone would say that is “nearly always fatal.”

Edit: my bad, sick and tired as I wrote that^. Chessfag here not sure why g5 to g6 would be characterized as “nearly always fatal.” The chess generalizations are getting annoying to me

That would be a black pawn moving down the g file towards the white pieces and I have no idea why someone would say that is “nearly always fatal.”