The act of simulating what you'd look like if you were a bit more toned and slim. Usually this means sucking in your gut, or pulling up your tummy in the mirror.
ME: Hey, do you think this swimsuit would look good on me?
YOU: I think it could, but run it through the slimulator first.
ME: Ugggh, yeah I already did. I'll have to shed a few pounds to successfully pull it off.