Concept2’s vortex edge has two key design features: a smooth round edge (compare this to the sharp corners of a normal blade); and small triangular devices designed to swirl the water as it flows by, making it stick more closely to the surface of the blade. This has a positive effect only during the first part of the drive, when the water flow approaches the blade at a flat angle toward the edge. At this point, the blade is traveling in a forward-sideward path relative to the water. After that, the blade at first moves backward and creates drag before it travels again forward-sideward, now back toward the boat when the part of the blade with the vortex edge becomes the trailing edge.
So theoretically there are compelling reasons to use the vortex edge. But measuring its effectiveness is another matter.
I tried to measure the difference by running tests with identical sculls, one set with the edge, one other without. The differences in force patterns and times rowed over certain distances ended up being as large as the variations you see when you repeat trials. The difference is also so minimal that, when blindfolded, rowers can’t tell the two apart. But even if the advantage is not entirely clear, there are practical benefits to the device: protecting the blade edge from physical damage.