How does the drag-to-select box (marquee) work?

Press the mouse button on empty space (or on the body of a row) and drag — a translucent green box appears, and every row it touches gets selected. Release to lock it in. You need to move at least a few pixels for it to count as a drag; a tiny movement is treated as a normal click. Hold Shift while dragging to ADD the lassoed rows to what you already had selected.