Gag curbs start at the jaw with a curb and then lift. Poll pressure is not totally removed as there is a similar action to the ring on the bit & top rein. When deciding between 3 rings and gag curbs they are mirror opposites as mentioned above. Gag curbs Pelham first and then gag.
If you have a horse that you have to pick up their heads first, but that action causes them to stiffen, locking the withers and back, we would recommend a gag curb to give you whole body flexibility through the withers and back and compression of step.