The space between the fixed surface and the movable surface[wing/ailerons, example] that little grove cause a air leakage,getting rid of that little gap helps the plane fly smoother and more responsive,a few think it stops fluter,I'm not one of those few. Bend the control a little further than max.then cover the gap with tape,or iron on covering,ultra cote,mono cote or?make sure the material goes down in the hinge line,like a V,or with the moving part straight,use a little clear silicon in the gap