I am playing with one in my hands right now. If you used an opening rein it will do one of two things:
1) swivel the whole shank so that the purchase gets turned and presses into the horses face and at the same time removes any slack out of the curb strap which then transfers pressure to the opposite side of the horses head , or:
2) flat out lifts the bottom of the shank out to the right which causes the purchase to dig into the side of the horses cheek...
I cannot get it to do either. Not on a horse.
If you use an opening rein, it barely moves the shank at all. It does not cause the purchase to dig in - not at all. The purchase is not on a swivel after all. One would have to twist the entire bit with a lot of force to shove the purchase into the horse's head. It does not remove any slack out of the curb strap.
These photos were taken in the last 15 minutes, using the exact bit I showed on Mia...who was quite patient about it all...but then, I had promised her hay pellets when we were done:
The last couple were pulling at an unrealistic angle since my arms are not long enough to get a 75 deg angle on the pull. In all 3, she was turning her head in response to the pressure, so I had to snap the picture quickly and hope for the best. The bridle was not perfectly adjusted since I normally use that bit now with Cowboy, who is a 13 hand pony. I let it out a few holes and figured it wouldn't matter for 5 minutes.
The purchase does not dig in. Indeed, the leather loop holding the bit to the bridle presses in more than the purchase does, even at a rest. Nor does the curb strap tighten, although that curb strap is set too loose for Mia...but no, it was not tightening. An opening rein is an OPENING, not a tug of war. An opening rein is done with fingers applying any pressure, if needed, not arms fighting with the horse. However, as you do
pull, it tends to slide the bit thru the mouth far more than it twists the purchase and digs into the head. If any thing, it seems to pull the purchase AWAY.
A single joint curb bit will never be my favorite if only because it has no rollers or rings for my horse to play with. But I cannot recreate the effects you describe, and my horse isn't the only one that moves fine in one.