Here is a short video demonstrating the action of the Myler Pelham with the 41PB (ported barrel) mouthpiece, which illustrates Myler's patented "independent side movement" very well. As is plainly visible in the first video segment, the entire mouthpiece rotates, bends, and folds about itself, creating a very flexible, fluid motion in the horse's mouth. Traditionally, a Pelham is an English leverage bit designed to be used with two sets of reins (one on the snaffle ring and one on the curb), but this is not necessary if only curb action is desired, and indeed, I find that this mouthpiece is more favorable when paired with a curb than as a snaffle.
In the second video clip, I have placed the Pelham in my mare's mouth and adjusted it as I would normally ride (yes, the extra curb length should ideally be cut off, but I digress). It was difficult to photograph--and even harder to film--the action when on the horse, so I will attempt to explain the two pictures.
One point that is often made against these sorts of bits is that the edges of the mouthpiece at the bottom of the port are squared off, albeit smooth. The concern is that the small surface area here could be damaging to the tongue. I found, and attempted to photograph, how the bit rotated and, when engaged and applying downward pressure, only the flat, non-squared portion of the mouthpiece was pressed up against the tongue and bars. This effect may be somewhat dependent on how the bit is adjusted, but my experimentation was quite sufficient for me to declare, at least in this case, that the squared edges are a non-issue. (You might also note that in the first photo, in my efforts to get my camera in place and hold the mare steady, I have pulled the bit sideways through the mouth--when riding, so much lateral pressure would not normally be applied, so the bit would sit more centered. Additionally, please forgive the chewed grass chunks!)
In the second photo, I have attempted to illustrate the action of the port. Again, the cannons of the mouthpiece have rotated so that the flat part is parallel to and against the tongue. The tongue is filling the gap in the "tongue relief" section, where the cannons curve upwards into the joint and the port arches further. The top of the flat port has come into contact with the palate, which is naturally low in all horses. However, the primary force of the bit at this point, with a moderate amount of pressure on the rein that would mimic a normal riding cue, is directed downward still, against the tongue and bars. While the port is hitting the palate (and disbursing pressure over a wider area than a rounded port would), it is not pushing up against it so hard as to create undue force, and the mare seemed completely unperturbed. The curb is hobbling the bit to prevent the port from moving further and, indeed, this is a relatively low port to begin with.
Cues from one rein are easily isolated, so direct reining is easy and clear with this bit. In summary, I knew there was a reason I liked this mouthpiece....