For a block repeat, you take your motif or group of motifs, bounded by a real or imaginary box, and line up the boxes straight up-and-down and side-to-side. But for a half-drop, every other column is staggered. (See graphic below.) The half-drop draws the eye diagonally in both directions and creates a pleasing balance — that’s why it’s the most common type of repeat. Above is the half-drop at its simplest — from the collection of myglorydaze on Flickr.
P.S. Sorry posting has been slow lately — life is kicking my butt! (Nothing bad — just busy.)