Oh no, not this again.... . The only thing the water does is prevent slippage of the fly line backwards. What "loads" or better still bends the rod is the amount of line in the D-loop is the conclusion and there was a heated argument when I asked a simple question of rollcasting upstream some years ago...There was a very long thread a few years ago about this... James (caster extraordinaire and brain-the-size-of-a-planet) proved pretty conclusively that the rod is NOT loaded due to water friction in a roll-cast. I seem to remember that there were some nice videos of him tying the end of the line to his trainers and roll-casting on a slippery tiled floor.
What water friction (stiction?) does do for you is allow a dynamic D-Loop to be formed.