I have heard about what the original poster is referring to. I have never seen it done, but the concept makes sense.
By crossing the reins, you get to add some direct reining to the neck rein. If you are going left, the horse will feel the rein on the right side of the neck. In addition, the crossed rein will put pressure on the right side of the bit, "pushing" it to the left. The idea is to do this one handed. Done right, the pressure is consistent - all on the right side cueing the horse to turn left.
As stated, never seen it or tried it. If set up correctly, the concept makes sense. In practice, however, it might be hard to do correctly. From what I have read it does work for some people.
Plus, the crossed rein is not only going to put pressure on the right side of the bit (if we were turning to the left) even if you have a snaffle in the mouth because the pull from the rein is coming from the back and right and not coming strictly from the left side as if were were direct reining. So there are TWO cues going on that is going to confuse the horse. And again, not to mention the fact that you should never pull so hard in a neck rein one-handed that it causes bit pressure. That's not the purpose of neck reining.
If your horse needs the reinforcement of a direct rein (and they do in the early stages of neck reining training), why would it be so hard to simply lift your inside hand for a direct rein cue reinforcement rather than crossing the reins and making a confusing mess?