Good question! Scandinavian or "Scandi" fly lines are actually shooting heads with running lines rather than full spey fly lines. Specialized casts that use them to their best advantage were developed on the big rushing rivers of northern Europe. A lot of the Scandinavian rivers have extremely restricted backcasting room, so traditional spey lines evolved into short shooting heads. A short, heavy shooting head requires minimal backcast clearance, and is easier to lift than a long-bellied spey line, especially if it is a sinking line. One of the main differences between Traditional Spey Lines and Scandi Shooting Heads is that Scandi heads require an attached running line, so you must strip the running line in to the shooting head before you can cast. This can actually be an advantage for streamer fishing, since stripping gives swimming motion to your fly as you prepare for the next cast.