The compression of signatures before sewing is an interesting, important, and poorly understood aspect of bookbinding. Many people immediately assume beating flattens the pages. Although this is partially true, the situation is more complex. Below is a brief introduction.
Both images, page 126 of John Marshall The Life of George Washington Philadelphia: C. P. Wayne, 1804. On the top: beaten, edges cut and bound in full calf. On the bottom: unbeaten, uncut and boarded. Courtesy The Library Company of Philadelphia.
These two images, shot in raking light with a flashlight, demonstrate some of the effects of beating. The book in the bottom image is from a boards binding and is unbeaten, but has been pressed. The book in the top image has been beaten: it clearly demonstrates increased textblock undulations as compared to the bottom image, where the undulations are much looser. Baxter, in 1809, mentions that beating makes the leaves “smooth and lie close together.”[1] Beating compresses the pages, smooths their surface texture, decreases the punch from the type, but it does not generally flatten the page overall. It usually does the opposite for the text area.
Pressing complicates all of this, but generally compresses the thickest parts of a textblock composed of handmade paper. Depending on the structure, time period, and nationality, bound books could be pressed up to six times: before sewing, when sawing in, during backing, while ploughing the edges, when applying spine linings, during edge decoration, and after pasting down the end sheets.
Depending on the techniques employed while beating, the margins of the page can get flatter, if beaten more, while the printed portions of the page simultaneously become more undulated. A useful food analogy is is to imagine pounding a veal cutlet. The meat moves outward in all directions from the blows of a hammer, as well as getting thinner. Careful control can direct the movement, however. Beating hammers usually have some “belly”, so depending on how much force they are used, impact a smaller or larger area of the page.
Beating causes the textblock undulations to become more pronounced, and the pages to lie in closer contact with one another. This has the effect of helping to lock together the leaves, much like inserting egg crates into one another. Much more information is available in my article, “Beating, Rolling and Pressing: The Compression of Signatures in Bookbinding Prior to Sewing” in Suave Mechanicals: Essays in the History of Bookbinding, Volume 1. Ed. Julia Miller. Ann Arbor: The Legacy Press, 2013. (pp. 316-381)
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1. John Baxter The Sister Arts (Lews: Printed and Published by J. Baxter) 95
Peachey Conservation 
I’ve never heard of beating before. Are you just hammering the face of the text block?
It can vary from just beating a inch thick section of the book, to shuffling the signatures within each section and beating both sides.
I am unsure of the function of beating the text block as a whole. However I have found it useful when resewing to beat the spine edge with a weighted stick to form the signatures around the sewing threads and remove excess air from the signatures before pressing. I do not believe that beating in that context causes cockling.
I you are interested in the function of beating the entire text block before sewing, I would suggest reading my fairly comprehensive paper, “Beating, Rolling and Pressing: The Compression of Signatures in Bookbinding Prior to Sewing” in Suave Mechanicals: Essays in the History of Bookbinding, Volume 1. Ed. Julia Miller. Ann Arbor: The Legacy Press, 2013. (pp. 316-381)
I agree, beating does not cause text block undulations, but it does exaggerate them.
What I am unclear about is how exactly the undulations come to be, and would love to get a letterpress printer to help try and reproduce this phenomena.