Bill Tindall, E.Tn.
.....The plastic sheet got stuck to the glass and the abrasive sprinkled on the plastic? If so this is exactly what is optimum. The hard particles will stick/embed in the plastic and become immobile. The stone will move and the abrasive will largely not move.

People unfamiliar with how the abrasive works are surprised that the case iron substrate for diamonds never wears significantly. The diamond sticks to the cast iron and does not move relative to it as the chisel or whatever is slid across the surface. Hence, little wear of the cast iron.

Obviously with glass as the substrate the abrasive rolls around between the two hard surfaces and abrades both of them. Not the optimum situation. Because both surfaces are hard, large particles of abrasive are subject to crushing.

To be clear, I absolutely understand trying something just for the curiosity of trying it, for example flattening a 6" stone if for no other reason than to try it. This kind of activity brings joy to life(for many). The risk is that someone will read about flattening a 6" stone and think they must flatten it for their pocket knife. My original comment was to initiate a discussion about when flattening might be necessary.