Michel Cabanac observed 2 directional flow in the veins leading to and from the brain and skin. Evaporative Heat stress was implicated in completely reversing the flow from the brain to the skin back to the brain from the skin.
I too have observed this two directional flow in a simple tubular model using food colouring and salt solution to cause a downward density flow and a return salt free water flow, clearly showing the turbulence generated by the movement of solutes. Termed as solvent molecular drag, in other words for the denser solvent to move through a tube it can only do so by dragging on the molecules throughout the entire tubular experiment.
I put it to Michel that the evaporative heat stress generated by exercise in a healthy person alters the density of the blood at the point of evaporation and this denser solution is dictated by gravity to move in the direction of the ground. When standing that direction is down through the path of least resistance and in this case the path of least resistance is through the vessels that pass through the skull to the brain.
In the case of the main veins from the brain it is plausible to suspect that blood can flow in both directions due to backpressure from a narrowing of the veins.
I have written to Michel and hope he will be able to add some thoughts to CCSVI.