This is a really cool concept called dragging the shutter or long exposure photography. If the shutter stays open for a long time (several seconds in the photography world is considered a long time) anything that emits light while the shutter is open will be captured in the photograph. If that light source is moving, that's exactly what you see. The reason you really can't see me behind the sparkler is that I am not emitting light and I am moving. This essentially makes me almost "invisible". A flash was fired to throw light on to the couple. Of course, having a crazy obscure talent of being able to write backwards is a huge asset when you try and do this with words!  It's amazing how fast 3 seconds goes by when you are spelling backwards with a sparkler at night!