From my trainer's perspective, flexing the neck from a standstill is something you don't want to do all that much, especially if you aren't doing it correctly. Correctly means that the horse does NOT tilt his head when He brings it around. many times you'll see a horse doing this excersize and he will tilt his head so that his nose comes up more toward the rider and his ears away from the rider. He's pivoting on the axis of his poll.
You want him to flex around such that his long nose remains in as vertical an alignment as possible. Then you get the stretch through the whole neck, and not just him pivoting on the poll.
So, first of all, to get the stretch benefit, flexs must be done correctly.
Secondly, if you do a lot of flexs in the more common way with the horse's head going downward and twisting around (described above) the horse is learning to lean even narder on the front end. It is , in effect, disconnecting the rein from the hind end. The rein should always have a connection to the hind end of the horse.
How? because it should always be mentally asking the horse to prepare himself to do something. To be prepared, he must have his hind end engaged, and be rocked back onto his hind (depending on what you are asking him to do).
But, I get ahead of myself.
When you do these flexions, does your hrose tilt his head as I described? Can you , by lifting the rein a bit more, have him rock his body back onto his hind? Can you get him to disengage his hind from one of these flexions? Doing that means that you tie the rein to the hind quarters.
First , you are asking the hrose to follow the rein around as you lift it . He should flex and follow the rein, and NOT be leaning on it. Then, lift the rein a little bit more and see if he won't rock back a bit and step that inside (inside meaning inside of the curve his body is making) leg under his belly and kind of step over. Release and praise. you just connected the rein to the hind end.
Eventually, just lifting the rein will get him thinking that maybe he should prepare himself for a turn . If he steps under himself with that inside hind, then the outside hind, the next step will be the inside fore, stepping over in the direction of the turn you are asking.
I apologize for this long winded and not very clear description. see if any of this makes any sense to you. Sorry, I am not always good at describing in words. And a terrible typist, to boot.