There is no leverage. It functions very similar to a full cheek (with keepers) in that bit is kept very still in the horses mouth.
Regarding single v. double joint, it really comes down to what the horse likes in its mouth. My last horse had a fairly thick tongue and low palate: he preferred a French link with a flat plate. He absolutely hated anything thick, or with a lozenge link (Sprengers were a hard no for him). He tolerated a single joint, but would definitely let you know if he felt the joint on the roof of his mouth.
Yup. No leverage. First used one on a horse that had a shallow mouh, large palate. He went pretty well in any snaffle but he liked the Baucher best. I use it on my present horse because he likes a quiet bit. He chucked a loose ring around constantly. I rode in a clinic with Hilda G, and when someone said it was a leverage bit, she demonstrated on my horse why it was not. She also commented that it was a good bit if your hands were a little unsteady.
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My guy tends to like the stationary side - full cheek/ eggbutt/ baucher. We just went with the baucher since the eggbutt I have has normal rings but looked HUGE on the side of his mouth - plus they interfered with the bridle. The baucher just looked better on his dainty ottb face. I generally buy double links, whether french or lozenge, and only get single joint if it’s for a specific horse.