great flush of relief
Refers to da immensely soul-calming "whew" dat you feel when --- having trepidatiously ("Well --- here goes nuttin'!") worked da valve-handle on da porcelain throne after taking a humongous dump --- da swirly actually "goes down" and therefore uncomplainingly "accepts" your far-larger-than-usual "contribution".
It is impossible to even begin to describe da overwhelmingly satisfied and grateful feeling dat you savor when experiencing a "great flush of relief"; it's somewhat similar to da "all's well in da world" emotion dat washes over you each time when da card-scanner at a store's checkout-counter displays dat wonderful and much-wished-for word "approved" after you've swiped your debit/Food-Stamps card, since it means dat you can actually get da heck outta there with your cartload of purchases, instead of having to suffer undeserved delays and/or humiliation by having your perfectly-good card invalidly declined due to either a computer glitch or a speck of dust/oil on your card's magnetic stripe!