Ready for the river

I've enjoyed looking over your flies in this and other articles on here. Can't quite figure out the benefit of the disc at the rear of the tube? Surely any turbulance created from the front disc is enough to move the materials? Is the rear disc simply holding up the wing a little higher than a bead or cone or dubbing knot would?

Also I have seen a pattern of yours elsewhere which incorporates a disc the wrong way round at the rear to slow the swing in cold water by acting as a brake of sorts. How effective have you found this and have your tried more than one backward facing disc to add to that braking effect???