Johan Bosman, JS Chief Aerodynamicist, explains: Whenever there is a difference in load on the wing, a vortex will form with excess drag as a result. Flap fences reduce the vortex size, because the pressure difference is “capped off”. The flap fences on the JS1 Revelation are fixed to the flaperons, not the wing root, as the load and pressure difference is higher on the flaperon.
There is however a trade-off for fitting flap fences, and in this case it is the added wetted surface area of the fences and interference drag. Recent oil flow experiments on the root with flap fences showed good flow lines, with no separation or vortices forming on the surface near the fences, which indicate no added detrimental effects. Whether it is truly beneficial to add flap fences is still a good question, but a current CFD study might shed light upon the problem in the near future.