“Back in the day” when folks started getting more precise in their expectations from Horse, they started studying bit styles and design. Why was “this” bit more used for this activity than that bit. I came up in Western riding with some English thrown in. At the time, the “better” horses went with a more vertical headset, with the “straight-up” California spade bit horse being the epitome of perfection! They actually still had folks training that stuff with years of work, back then!
I attended a week long clinic with a talk about bits that lasted a few hours. It was like the light bulb went on for me!! Bits were “form to function” as explained in detail by the trainer. Spade bit cheeks and mouthpieces were totally spelled out, what the rollers, spoons, thickness of mouthpiece did as a whole in the mouth of the WELL-TRAINED horse. High ports, wide but not high ports, long and short cheeks, reach (above the mouthpiece) choices affect what the horse feel. Cheek shapes change how the mouthpiece hangs from the bridle. With Western loose (but not thrown away like now) reins, bit functioned to the design, rider usually did not affect bit much except in stopping, though neck reining with serious pressure, did change bit in the mouth. Most bits were the H style, solid mouth, fixed cheeks.
Trainer speaking advised us to see how bits worked in the mouth, by laying mouthpiece across our palm, see how it hung. It was very interesting trying his many bits, learning how they worked in the mouth. Thickness of mouthpiece, width and shape of ports, angles of cheeks. Importance of proper width to keep horse happy. We also did some English bits, the common Pelham with various mouthpieces, snaffle bits with various sides, O, D, Full Cheek, Eggbuts, Weymouths.
Basically, thin mouthpieces are harsher, more bite into bars because they have less surface. Thicker mouthpieces are nicer with more metal on the bar surface. Bit with a higher reach (bridle loop above the mouthpiece) will exert more pressure on poll, harsher pull on the mouth from rein pull. Tight or loose curb chain also affects bit pull force.
There have been incredible steps in bit design over the last 20 years! So many choices!! However if staying with traditional Driving bit side like Liverpools, I expect to see the bit side vertical, aligned with horse lips. Seeing a cheek pointing back at the Driver, makes me think (right or wrong) that horse is pulling pretty hard. Some types of horses have a hard time going with head vertical, just not comfortable for long. So those type equines going a bit nose out, a bit with upright port aligned with cheeks, will be nagging this horse all the time with curb chain and rein weight pulling cheeks down.
This is where the forward port mouths confuse me, not hanging well on my hand. But horses do like them! Horses do their own thing despite science! Ha ha
I felt the price of the whole Clinic was worth it just for his talk!!