Here's what other sites are saying:
The Rottefella Powertubes allow the skiier, depending on their weight, boot size and ability level to customize the level or resistance and activity of the NTN Bindings. Powertube colors/stiffness: Extra Soft (white) Soft (green) Medium (blue) Rigid (red)
The new NTN telemark binding system from Rottefella gives you the ability to adjust binding flex by selecting different Power Tubes. The power tubes affect the stiffness of the binding vertically and laterally. For example, a pair of stiff power tubes will be more difficult to flex and more rigid side-to-side.