I actually went to a Dominican stylist for about 18 months(moved to a place where the only Dominican salon is too far to be a good option, and working out makes straightening pointless) b/c I couldn't get to the person that I trusted to relax my hair(too far away). My hair did so well (and she gave me a great cut with long layers) that I kept going and wound up growing out my relaxer. I never let them relax my hair b/c I'd had bad experiences letting people I didn't know do it, so it started as a way to stretch out the relaxer.
I have to say, my hair did great, was never heat damaged, and my ends stayed perfect and retained length. I'd always had long hair, and it grows well, but it did better than usual I'd say. They cut my hair maybe to collar bone in late January (getting rid of hair damaged by another stylist) and by end of summer it was MBL again. (oh, and I got cuts in between that time too)
I know they aren't all the same, but if you have natural hair and want to get it straightened, they are the best at that that I've been to recently. I had no issues (as in natural being better or worse than relaxed) with the relaxer other than the fact that it was hard to keep people from overprocessing me.
I'd used their products before b/c they were low cost and seemed to work pretty well, although now I used more oil/butter based products.
I don't think there is any magic formula to make hair long but if your hair is blown straight, it doesn't tangle and it's easy to keep moisturized. The whole issue with curly or kinky hair is that it's hard for the oil that your follicle produces and for what you add to travel down the hair shaft. So if your hair is straight, anything you apply is easiliy smoothed onto the entire length of your hair. When it's not, you have to really work the product in there.
Also, when you comb or brush, you can just glide through, and the only hair that comes out of your head is what is shed. Nothing gets broken off.
So for my straightened hair, I could comb out shed hair morning and night, wrap the hair up, and yeah, it's easy to see why my hair retained all of its length. No scientific mysteries or magic there.