This is on topic, really.
Ever see that Simpson’s episode where Bart takes up ballet? Remember the mad sign-up for gym options? Push ups? Lap running? Tethered swimming? I think that Springfield Elementary is a breeding ground for triathletes.
Back to the topic at hand… After a (swim) coaching session, I fashioned some resistance training bands for drill work. Basically I copied these: http://swimzone.com/acatalog/Swim_Zone_SWIM_CORDS_35.html
(NZ Modular) only mine are way overbuilt.
Attaching one end to the ladder and the other around 1 ankle, I would swim away from the wall focusing on whatever it is that needs work - catch, finish, roll, hip drive, etc. I found that swimming with the cords really highlighted problem areas, and gave me an excellent feel for what I was doing wrong, as well as emphasized the proper roll (this is the advantage to only tethereing one ankle, as opposed to both as many systems you see for sale recommend). After swimming like that for a minute or so, I would remove the band from my ankle and swim freely for ~100. Once freed from the band, swimming seemed very effortless while at the same time extremely powerful. I was flying (for me). It was also very easy to feel the difference in my stroke, before and after band usage. After about 50 I started to lose the feel, but it was much easier to tell where and what was going wrong. Strap the band on the other ankle and get it all back again, with a slightly different feel (emphasis on rolling the other way).
Frankly, I am amazed at how excellent a tool these bands have turned out to be, and am at a loss to understand why their usage is not more prevalent. These have the capacity so clearly show flaws in one’s stroke and help to give the correct feel for proper swimming. Hip drive, roll, catch, pull, and finish were all highlighted. I think it even made my kick a little bit better, having been forced to kick against the resistance of the band. As an experiment I tried attaching both bands (one to each ankle), and found that it interfered with the roll but that it provides a kickass way to improve fly kick. I didn’t incorporate fly arms because my shoulder is still recovering from an injury, but I suspect that both bands would be an excellent fly-development tool.
The last thing I noticed was that the stationary/dynamic nature of the bands provides an excellent opportunity for external critique. In making these I have essentially created my own flume, or Endless Pool. I was out with my boy PowerGyoza, and discovered that I could very easily walk around him and analyze his stroke from 360 degrees, under water. Not only that, but because he remained essentially in the same spot, I could stand in front of him and watch him swim “towards” me at an ideal focal distance, for as long as he kept swimming. This provides an excellent opportunity for feedback, and even though we do not own an underwater housing for a video camera, gives a glimpse into what filmed flume training can accomplish. We’re going to run out and buy a few of those disposable underwater cameras and see how that works out.
Lastly, these things are big time fun! This is very different from lap swimming, and the intensity of the band pulling you back makes it a lot easier to maintain focus - 100% of the time. Then when you’re done, stop swimming and enjoy the ride back to the wall! 
I just thought I’d share my experience with these, as PG and I were able to learn a lot about our stroke in just 2 hrs using these. They’re extremely cheap to build, and have already proven their worth a thousand fold in ways that I never anticipated.