Essentially mewing but with your buttocks. You squeeze your glutes at all times to shape it into a plump babypumpkin carved from marble. Many clenchmaxxers have expressed feeling like they're in a higher plane of existance from performing this, not unlike gooning.
"Oh, you've been mewing, ropemaxxing and jelqing for the last two years? But you haven't started clenchmaxxing yet? Never gonna make it"
When one has redlined their edge, and is deep into the euphoric gooning state, you begin to squeeze the base of your bird until the mushroom reaches maximum plume. You have successfully clenchmaxxed when the post nut clarity brings you down to earth and you realize now you have to patch the gaping hole in your drywall from your atomic chode dumper.