I would like to highly recommend this article in particular, and this website in general, as a source for Dressage education. I am going to quote directly from the article here:
"Held Up with Force
Some people think that I rant about rollkur riding and overbent necks. But if there's one thing I really really hate, it's when strong riders with a double bridle habitually pull the neck as far up and back as they can, in the name of collection. This is the most detrimental posture a horse can take with a rider on his back. "
I choose this quote from the article, because it is long and detailed and not all will want to read it. But this quote says it ALL. If you see a horse that is being ridden this way, the "tell" will be this: watch them in their "collected" canter-the horse's mouth will gape at every downbeat of the stride as the poor thing pivots off the 100 lbs of pressure the rider is holding on their mouth. Their jaw is doing the work their hindlegs should be doing. Sometimes the horses will almost offer this arrangement because their hindlegs are not strong enough to perform what it is their rider is asking of them. A wise rider will recognize this evasion as the horse screaming that he is simply not yet strong enough, and, take the time to carefully strengthen the horse's hind legs in the classical way. Or, have the vet out to check for problems, if it is a new evasion from a horse that does already know how to carry the collection correctly behind. If the riders don't care that they can't feel their fingers when they are done riding, then, reading this MIGHT make them care about what it is they are doing to their horses. Or, maybe they won't care. Rarely will judges be able to see this, or probably more likely rarely will they realize how big a problem it really is and instead reward the high poll and flashy front legs, and these horses usually score well, high 60s even low 70s at times. But when a REAL judge comes along, who absolutely know what absolute elevation looks like, and scores them lower, the rider will simply chalk it up to the judge being "grouchy".
It is important we as riders learn to recognize what is correct and what is not. We have to be very disciplined with ourselves and not be seduced by the siren song of advancing up the levels before we, and our horses, are ready. Below are photo examples of a correctly balanced canter where the horses are not pivoting off the bit for balance, one more collected, one less collected, but both in the correct balance for their development. Both horses are in, or approaching, the downbeat of the canter. Neither horse is gaping its mouth, nor is the rider carrying her hands in an artificially high manner with excessively short reins in order to force the poll to remain the highest point. The poll is the highest point correctly, because the hind legs have lowered to accept the weight and lighten the forehand. No doubt we all have our bad moments as riders, and not every photo will portray you and your horse in the best light, but if most of your photos look correct most of the time, you can probably bet that for the most part, your horse is working in the correct way. It is not easy to ride correctly, progress will be slower sometimes as you wait for your horse to gain the necessary strength, but it will be worth the wait. I promise.