Simple solution really. Make sure you soak your biots first so they are not brittle. Then, attach the biot by the tip with the small notch that appears on the wide end either facing the hook eye or the rear of the hook - here is where you achieve a smooth or rough biot body. If you attach the biot with the notch facing the hook eye (to the right for right-handed tiers) you will achieve the rough look with the 'spine' as you call it standing up. If you tie the biot in with the notch facing the rear of the fly (left for right-handed tiers) then you will achieve a smooth body. Also, make sure to apply a very thin coat of cement to the hook shank prior to wrapping the biot.
Here is a pic of what I mean by the 'notch' facing to the right for an end result of a rough body:
Here is an example of what results by tying the biot in with the notch to the left:
Best of luck. The key to success is assuring your biots are pliable by soaking them first. A small sponge similar to what they make for wetting postage stamps works quite well. Have several biots soaking prior to your tying session.