Dropping back is like riding a bike (but not because you never forget how to).
Students frequently ask why they should try to stand up and drop back before doing the backbends of the Intermediate series when those postures seem so much more approachable. I have a very long answer and about fear and trust and big picture ideas that includes the bicycle analogy (which falls flat on some Russians I know who never bothered with bike, what with all the snow, so I already know it’s not universal).
When I learned how to ride a bicycle I took a pretty all-or-nothing approach. I practiced coasting and tried out the training wheels, but didn’t do much more to prepare for the big day when I expected my Dad to be holding me and he didn’t and I fell and everything turned out fine. The next day I tried again and maybe I fell a little less hard or maybe I ate it, but either way I was out on my own. It wasn’t until after I had a pretty good feel for the general mechanics of balancing while steering and pedaling or braking that I attempted to specialize in any one aspect of it. Hell, I would forget how to do any or all of those pieces if I got distracted or had too much fear. But time passed and I built up reflexes and soon I was able to focus my attention on shifting gears up hills and skidding my tires as I braked in a swooping arch (and ‘soon’ in childhood bike riding is much like dropbacks, it may be days, weeks, or years, depending on your aptitude and how much you practice).
Riding a bike for the first time is scary. Sitting on the bike while stationary and practicing braking or putting the bike on a trainer and practicing pedaling will not truly teach you how to make it to the tour de france. It might give you some little hints to keep in mind, but in reality there isn’t going to be space in the mind for the little things until you have figured out the big picture. Once you can ride a bike, steadily and completely, without help and with without too much fear, then you can worry about more detailed maneuvers.
The real message: drop backs do not have to be that big or hard or scary. Kapotasana is another story altogether for many of us and each Intermediate backbend in teaching the body something specific and necessary to be able to turn the work of drop backs into Kapotasana. Dropbacks are the foundation on which we build a more acute awareness of back bending in the beginning of Intermediate. Without this foundation we may gain a lot of great information, but be left unable to put it together functionally or usefully.
Other schools of Yoga use these postures for different purposes (entirely valid and useful purposes). In the Ashtanga method every posture is intended to teach us something specific, to be used as the foundation to build on later. They each come as the appropriate time in the learning curve. Training wheels are encouraged (dropping back with your teacher) and you can coast as long as you need to before putting your feet on the pedals (just plain backbends from the floor) but without actually trying to coordinate everything as once, specializing in the parts won’t be as effective.