diff --git a/R/revealjs_presentation.R b/R/revealjs_presentation.R
index 047bda3..d93e617 100644
--- a/R/revealjs_presentation.R
+++ b/R/revealjs_presentation.R
@@ -94,6 +94,21 @@
 #' The function [slide_timing()] computes these totals directly from an
 #' `.Rmd` source file without rendering it.
 #' 
+#' ## Vertical content centering
+#' 
+#' Slides with little content look lost when everything is glued below the
+#' title bar, while packed slides must start at the top. reveal.js' own
+#' `center` option cannot express this -- it also vertically centers the
+#' headings. Instead, the body content of each slide (everything after the
+#' heading block except speaker notes) is grouped and, whenever there is
+#' free space, centered in the area between the heading and the footer:
+#' slides that are too packed simply start directly below the heading and
+#' overflow downwards, exactly as before. Centered content never reaches
+#' into the footer; content too tall to fit above it may, as it always
+#' could. Heading-only section-divider slides keep their own centered-title
+#' layout. No configuration is needed and nothing changes about how slides
+#' are authored.
+#' 
 #' ## Chapter numbering
 #' 
 #' Like in the old Google Docs slide workflow, chapter headings (`#`) are