swim fins mystery

In my experience, fins can help give you amplitude and speed when learning as a beginner for strokes like freestyle of fly, like a pull buoy will help you float.
Also, (and in your case) for freestyle especially, fins can help you work your leg muscles and technique a bit in the same way that paddles do for your arms. The first thing is therefore to make sure you have some technique before using the "tools" to avoid injury or overdoing (like your friend).
Just like paddles, the fins accentuate your body's actions and therefore reveal the details of how you swim.
Based on your description, it seems that you have good flotation, but that your kicking technique is a bit wide or "all over the place" and therefore when using fins this is accentuated and feels weird (legs are heavier with weight of the fins and water resistance is increased). If you are using the fins to really work the muscles, then I recommend starting with only kick laps when using fins and you'll notice that your muscles (more than breathing) will be challenged. You will also improve kicking technique (remember from the hip, not the knee).
When swimming regular sets (no kick board), normally the fins should allow you to kick a bit less (frequency) and go just as fast - unless your kicking technique is wrong and therefore since the fins make the kick more prominent than the arm strokes, it slows you down. It seems this is your case. When swimming without the fins, your stroke technique (which I am assuming you've worked on a lot more than kicking technique) compensates for you legs and therefore you have a good quick swim and good endurance thanks to training and proper breathing (and maybe good natural flotation). However when the fins take over, your inefficient kicking technique undermines your stroke and slows you down. I recommend working on kick technique in workouts and not relying so much on kicking in races.
YOur friend on the other hand, seems to generally use her kick very little, (though it may be a pretty good kick) - and might have less natural buoyancy than you. Therefore when she added fins and actually used them,she flew away with an efficient kick! However, the danger is that swim fins really make you work and increase speed and people often feel like legs and arms should go at the same pace. This is not necessarily true. Elite swimmers don't kick that much really compared to their strokes because it is taxing and tiring and the return isn't as high as proper stroke technique. They mostly use kicking as balance.
Therefore, your friend needs to work on kicking more, but also on separating the top and bottom of her body. This requires a bit of coordination, when it does not come naturally.
This is a good 200m set to help coordination:
50 free kick
50 X (arms slow, kick fast)
50X (kick slow, arms fast)
50 Free
Do this 2 or 3 times for several work outs and it will help (you and her) to better feel your body's elements.

Hope this helps, enjoye the water

joe

A few things I notice.

1. You can see that you may not have a very effective kick.  Fins are supposed to help you and with your experience they did nothing.  Your ankles aren't flexing enough and your kick could be weak

2.  Fins are designed for helping you go faster then you normally swim.  It's like having a long run of 8miles when you are training for a 5k.   Go farther or harder then you would in a race so that your race pace feels "slow".  Fins make you feel faster and let your mind and body feel what its like to go slightly faster.  

3.  Don't feel bad about fins making it feel weird because it should.  For humans, fins aren't normal and its a different feeling just as paddles are for your hands.  Its a training tool designed to make you fitter and faster in its own unique way

4.  Start adding more kicking sets to your workouts and wear fins more often.  Try wearing fins for a whole practice (although you might get some bad cramps)

5. Look into trying or using scuba type fins that don't flex as much (US Divers makes some good ones) to help develop those hip flexors.

6.  Next time you are swimming with fins focus on kicking super hard and swimming.

Hope that helps! 

The Aqua Sphere AlphaFin Eva Foam fin was recommended to me by two different coaches because it helps with your body position more than just adding propulsion. The fins help keep your feet elevated and aid in learning to kick from the hips and not from the knees. They also help with muscle memory and keeping your toes pointed and decreasing drag. By having the correct body position it improves your form in general.

I had a bit of a cross kick which this helped correct and I feel like I'm starting to generate some propulsion from my legs correctly without creating drag as I was before or relying 100% on my upper body for the propulsion.

Just an FYI I don't claim to be an expert.

Thanks--will pass on the idea to my friend. I think in my case it may have been a matter of ankles not flexing enough and technique; also leg fatigue as I'd done a hard bike workout that AM. I don't think my kick was doing much, with or without the fins! Fins may have been too small as well--I may have have grabbed a kiddie pair as they felt really tight at the ankle. Honestly I rarely think about my kick--I've always been a distance swimmer and I don't really kick strongly unless I'm sprinting or doing OWS in rough water.  I'll ask the coach to take a look at my "normal" kick without the distraction of fins, or just kicking with them next time--he was busy last time helping a beginner.

I use fins because I was told they would help stretch out my completely inflexible runners calves.  It creates an active stretch.  
 

this was kinda answered earlier up-thread... but my question is kind of the opposite:  why is it so much easier for me to swim, and my whole stroke/body feels so much more streamlined and "tuned in" when i swim with fins.  is it because my kick is really weak?  or is it more of a balance problem/body positioning problem when i'm finless?

sorry, hope this isn't too much of a hijack. 

I don't use fins often.  I might not want to stop!  I have a weak kick and I absolutely fly through the water when I have fins on. Sometimes my feet start to cramp in the arches though.

I don't much see the point to using fins, but it sure is fun. Zoom zoom.

-eric