When someone preaches some truths, you use this phrase (usually in comment sections online) as a way of agreeing.
For instance on Facebook you may have:
Person A: "You should never blame a whole group of people for the actions of a few people within that group!"
Person B: Say It Again Louder For The PeopleIn The Back.
"...and Africa is a continent, so basically, everything you've said is bull and only a tree would believe it!"
"Say it backwards, Howard."
"What? What's that supposed to mean?"
"Say it backwards."
"...uhh..."