The plastic tubing used in the construction of these flies is very lightweight and flexible. This extra-lightweight tubing combined with a heavy brass or heavier tungsten cone gives this fly design a front-end weighted action. This often accentuates the variances in the currents giving the fly an action that settles in the calm spots and rises in the fast ones; giving the fly an up and down motion.
http://www.flyfishusa.com/fly-tying/fly-patterns/scandi-tube-fly/scandi-tube-fly.htm
Good info on components at the Welches, Oregon site.
SwittersB & Exploring 