We learn that the strength of a curb bit depends on the length relations between the upper and lower shank. This is true for curb bits as the rein is attached at the bottom and the curb chain in the top. For hackamores this is not the case because none of the affected straps are attached in the upper shank.
Instead, we have to look at the three points that will be charged when we stretch a hackamore rein. There is of course the bottom of the shank where the rein attaches, but also the two points where the nose- and curb strap attach.
The power that affects the hackamore's strength is related to these points. We can simply say that the longer the shank is, the sharper is the hackamore, and the longer distance between the nose strap and curb strap (point 1 and 2), the milder is the hackamore. To understand this, we need to turn the hackamore 30 degrees, as it sits on the horse when we stretch a rein.
Now we can recognize all images on curb bits that we've seen. The bottom line corresponds to the rein, and the other two sumbolizes the forces that affect the nose straps. The strength of the hackamore can be calculated as the ratio of the distance A and B. We can see, that if the distance between point 1 and 2 is shorter, the hackamore gets sharper.
If we let the hackamore rotate a bit more, we get other conditions. Here we can see that if we turn the the hackamore 45 degrees, distance A has increased and distance B decreased, which made the hackamore milder. So unlike a curb bit, the hackamore gets milder if we loosen the curb strap. We can discuss a long time at what strap length the hackamore works best, but I will leave that to you.