Presented here are three different types of curb bits: A Myler Kimberwick, an Argentine snaffle, and a long shank Sweetwater. Each has been adjusted on the horse exactly how I would do so if I were going ride. I am pulling on the reins from the angle that would occur in the saddle, so that everything mimics the riding condition as much as possible. The point of this video is to show the relative degree of poll pressure (or lack thereof) that occurs with each of these bits. Poll pressure is often touted as an inherent component of the action of a curb (leverage) bit, but this is not necessarily the case, depending on the exact mechanics and design of the bit, the adjustment of the curb chain and headstall, the degree and angle of contact on the reins, and the horse's head position. While it is difficult to capture on camera exactly what is going on, I have tried my best, and am providing this description to accompany the visual.
First, please note that in order for poll pressure to be an action of the bit, the cheeks of the headstall must tighten, as they are the connecting pieces between the bit and the crown (poll). If the cheek pieces are slack, or if they do not further tighten from their default setting when rein pressure is applied, then poll pressure cannot be part of the action of the bit.
For the Kimberwick, note that the cheek pieces grow very slack as increasing rein pressure is applied. What is happening is that, while the bit is indeed rotating and applying curb pressure, it is also rising in the mouth. The shanks are short and the curb activates slowly enough to allow the cheek pieces to loosen (you can see them bowing out). Poll pressure actually *decreases* from default when this bit is activated.
Next, look at the Argentine. It is difficult to observe what is happening with the action here, and there is certainly no visible bowing of the cheeks, as with the Kimberwick. However, if you look closely, you can see that the cheek pieces remain completely in place and do not appear to have any additional tension placed upon them. The purchase ring of the bit slides freely over the cheek piece's attachment--it does not grab and pull it forward, so the crown is not activated. I did place my hand under the crown piece of the bridle while pulling firmly on both reins, and I was unable to feel any tightening, even with considerable force applied to the bit (and a thankfully tolerant horse).
Finally, consider the Sweetwater. This is a hefty, severe curb bit, so I did not pull as hard on the reins. Still, while it is hard to visualize in the video, you might note that there is indeed some tightening of the headstall's cheek pieces. I placed my hand under the crown and was able to feel poll pressure that increased as I applied more force to the reins.
In summary, poll pressure is certainly a component of the action of some curb bits in some scenarios, but it is not a foregone conclusion and its severity depends on multiple factors.