You need a bounce surface, or they do nothing.
If you have normal white ceiling, and maybe a wall or two either side, then if you point the Stofen up, it fires most of the light to the ceiling and a bit to each side and behind. This bounces off the ceiling etc, and this then becomes the main light source, all nice and big and soft.
As such, that is very similar to regular bounce flash without a Stofen, which is good - soft light that spreads all around and fills a large area with relatively even illumination. But as the light is coming from above, it gives shadows under eyes and chins. So in addition, what the Soften does is send a small amount of light directly forward, which lifts those shadows and puts a sparkle in the eyes. It works really well in you 'average' room.
A bounce card does much the same thing. As do many of the similar on-flash diffusers like the DembFlip, Fong etc.
Any form of bounce flash uses tons of power though.
Edit: the end result is effectively a three-light exposure, with each component nicely balanced. If you've set the shutter speed to expose correctly for the ambient light, the background should be correctly exposed - that's one light. Then the foreground and middle ground is lit by the bounced flash from the ceiling, and finally the third light source is the direct flash component from the Stofen. Cool eh?
