french link the wide flat part of the plate/lozenge sits on the horses tongue, Dr Bristol the thin edge of the plate/lozenge sits on the horses tongue which ovbiously puts more pressue on over a small area and hence is more severe.
However compairing that to some other bits I'd have a hard time saying they've very severe myself. But that's just me.
i think a Dr bristol sits at an angle and applies hard pressure on the tounge whereas a french link lies nice and flat on the tounge and spreads pressure nice and evenly. i thought that with a Dr Bristol you shouldnt use it unless adviced to by an experienced instructor?!
A French link has a rounded middle section (either a rounded plate or a peanut or a bar) which sits flat on the tongue. A Dr Bristol has a sharp edged centre piece that sits at an angle to the tonge and obviously applies pressure over an edge, back and down against the tongue rather than over a larger, smoother, flat area like the French link.
The plate on a Dr. Bristol should lie on the same angle as the tongue (top edge further back) rather than against the lay of the mouth as incorrect placement makes it very harsh indeed.
French links are legal for dressage, Dr. Bristols are not.