I'll have to ask k_ellis, do you feel any resistance when sweeping your fin sidways through the water?
Honestly I have not even paid any attention. If their is any resistance it would probably be minimal. I use them also in an aquarium which requires some fairly tight turns. I also find stopping then turning around and taking off an extremely easy concept compared to the blade fins out there that I can attest too.
Ive done some side finning as it makes cleaning the aqarium easier too and I can not report really any difficulties to speak of myself. Overall for such a tight space as an aquarium they have proven to me to be the greatest I have used yet.
In the open I cant really rate them in a current yet as I have not had the chance to place them in the ocean on a drift dive but this will be forth coming also.