Marquee selection
Start the drag on empty space — either inside a board or on the bare desktop between boards — and the band follows your pointer across board boundaries. Everything the band touches is selected, regardless of which board it belongs to.
Hold Ctrl to add to an existing selection rather than replacing it.
This is one of the places where DeskDrawer behaves less like several windows and more like one desktop. A cross-board selection can then be dragged, copied, cut or deleted as a unit, and keyboard shortcuts apply to it immediately.