The “roll-in” method is the description for what you do when you laminate dough for croissants, Danishes and puff pastry. Effectively you’re “rolling” butter into a flour-and-water dough. Personally I think of it as “folding” it in, but there you go. Who am I to argue with decades of established pastry lingo?
There’s no question that laminating seems more like a technique than a “mixing” method, though when you consider that one of the chief aims of mixing is to incorporate fat it all starts to make a little more sense.
So what does the roll-in method accomplish? By itself it’s an elegant way to maximize the process of “mechanical” leavening, i.e. the raising of a dough via steam power. Lest we forget, a drop of water transformed into steam occupies something on the order of 1400 times more space. Which makes confined steam a heck of a leavening engine.
Done well, the roll-in method creates over a thousand ultra-thin, alternating sheets of fat (usually butter or margarine) and dough. When heated the fat melts, freeing the dough sheets to push apart from one another through the action of steam.
The question often asked is: where does the water come from? The butter? Yes, though plenty of water/steam is released from the dough itself. In fact the dough supplies all the water that’s needed for leavening. A “wet” butter with a high proportion of water can actually harm the process, dampening the dough sheets and making it harder for them to separate from one another and rise. This is why experienced laminators favor fats like Euro-style butter, “dry” butter or margarine which have little-to-no water, and which create higher rising, crispier products.
How high can laminated doughs rise? Under perfect conditions, up to 7 times their original thickness. Granted that’s far less than the theoretical 1400 times, but then nobody’s perfect. Even under the best circumstances the vast majority of the steam we bakers try to capture escapes.