From Kayak Wiki
In a kayak, to propel the boat over a rock or ledge, and to land flat making a boofing noise Onomatopoetic. (Hence the name). Usually done in steeper rivers where the paddlers do not want to go deep.
BE CAREFUL landing flat from any height can hurt you badly!
In a canoe when the yoke snaps during a portage and lands on your head. Also onomatopoetic