Blades of the turbine have cuts which open cavities inside the blades. The stream of air or water comes inside the blade and presses inside too.
Thus the area of pressure of a wind or water increases almost in 2 times without change of a configuration of the blade.
Pressure inside the blade will not be compensated by pressure from above. On the contrary - they will be summarized.
Accordingly capacity of the turbine increases in 2 times too.
Rigidity of the blade does not decrease thus. Longerons and ribs provide stability of the blade and reduce its weight.
More complex jet variants of such turbine are possible. In this case the turbine will have the second cut on the bottom part and an additional plane inside. The stream of air or water will come through the top cut, then it will pass around of a plane and will be squeezed out through cuts from other surface of the blade.
This jet stream will increase capacity of the turbine even more.
More detailed information can be looked on a site.