The curved mouthpiece gives the horse room to swallow freely encouraging the horse to listen to the rider and it also distributes pressure out evenly. The part in the middle which looks like a roller is actually fixed, and it just covers the joint, evens out pressure, allows independent side movement, and also makes sure the bit is the same length both sides. I have no experience with them personally but I have only heard positive feedback about them.
Hope this helps
I haven't used that particular Myler bit but in my experience with their other bits, they are great for horses that don't need something very strong. I love the ones that I have used. I use mine mostly for dressage, flat rides, and light jumping.