A sword brush is a bit like a rigger or liner brush, but is steeply angled rather than pointed. You can paint an extremely thin line by using only the tip, or a wider line by holding the brush so that more of its hair touches the surfaces. No surprises then that it's also known as a striper brush.
By rotating the brush in your hand as you move it across the surface, and by lowering or raising it, you get fluid, calligraphic mark making. If you hold the brush loosely in your hand and move across the surface quickly, letting it do what it wants to some extent, you get a free, expressive mark. Great for branches in trees, for instance.