I ride in a snaffle, no shanks at all. So, I direct rein mostly, though a person can neck rein in a snaffle any time you want, as long as the horse knows the cue.
I think of flexing the horse as a way to reinforce his connection to the rein. I don't flex him at all if he is already lightly followoing the rein. Follwoing the rein means that if I pick up the right rein to go right, the horse turns his head at the poll, looks to the right and then he follows that head turn with his neck and body and thinks in that direction and goes that direction.
If I lift the right rein and askf or the kind of following the rein to the right, but the hrose stays looking forward, stiffens his neck and jaw and resists the rein, then he needs to be reminded about the rein and what it means . So, I lift the same rein higher and ask him to fles his neck around and to rock his body back a bit. I do not loosen my hold on the rein until he starts to willingly bring his own neck around, and I even look for his eye to roll back and indicate that he has given up the thought that was focussed out forward and is now thinking back to ME. When he stops pulling back on the rein, then you give him a big release. I also like the horse to step under and disengage, but if he changes his mind and flexes softly to the rein pretty quivckly, then I don't need to make him fully disengage his hindquarter.
I know , that was way more than you wanted to hear. Between me and USandpets you can get the general idea, no?