I can tell you exactly what it was like for me. When I have gall bladder attacks, I pass stones. That's what a gall bladder attack is for me -- a stone finally making its way from my gall bladder into the bile duct and then being passed out with much pain as those muscles and tissues try to dilate enough to accomodate the object being ejected. I've even found the gall stones in my excrement afterwards -- even the big ones, which are scratchy on the way out, with "facets", like some of those you see in pictures of removed gall bladders on this site... And this was before I had ever researched liver flushes.
I've also passed those same stones by doing flushes without any pain at all (more like a very slight internal tickle), so don't be alarmed. They can come out painlessly -- that's what flushes are for.