It's all a question of energies.
Bernoullies equation : (ignoring compressibility effects - as at normal windspeeds there is negligible compressibility) - states that energy in =energy out (ignoring heat due to friction).
The kinetic energy of the air flowing through the wind sock is transformed in part to potential energy (in raising the weight of the sock through a proportional height); and pressure energy brought about by the ram effect. The sock positions are infinitely variable as is the restriction to the flow of air through the core (within the limits of completely open and completely collapsed). Therefore the ram effect is proportional to wind speed (as we all know) but also proportional to the aperture of the restriction. An infinitely large aperture would cause no restriction to flow resulting in little or no ram effect, thus preventing a pressure differential from existing across the wall of the sock. The smaller the apperture then the greater the restriction to flow of air and hence the greater the ram affect for a given free stream air velocity. The ram effect being created by the sudden slowing down of the airflow due to restriction and hence (here's where bernoulli comes in) for the total energy to remain constant the remaining two forms of energy must increase. Hence the sock inflates and is lifted through a height.
