The procedure is a little tricky: 1) select the parent bone (by left clicking, or in the tree), 2) hold ctrl (cmd on Mac) and click one or more attachments (or click+drag to box select), 3) stop holding ctrl/cmd, 4) click or click and drag to draw the bone.
Pressing ctrl while drawing the bone does make Spine stop drawing the bone. It doesn't actually freeze, but it looks like a freeze so we'll improve that. 🙂
