I did a few weeks work experience at a major TB stud a few years ago. The level of restraint used on the mare varied depending on her behaviour during scanning an handling. When scanned, mares weren't twitched or tranquilised if it wasn't needed. When a mare was ready to serve, a pony teaser was allowed to mount to determine the reaction of the mare. Duting breeding, most mares were twitched and wore padded boots ont he back feet. The stallions are made to tease the mare before being allowed to mount. More difficult/nervous mares (or those with foals) were mildly sedated, lip twitched, front hobbles and back booties. I remember one particularly difficult mare had all of the above, plus six handlers trying to get her to stay still for serving (btw she was definitely ready to serve, but would kick at any horse within a mile of her). After throwing herself around to avoid the stallion (who was being very patient), he eventually mounted her, when she then laid down. I think this was a case of a sexually receptive mare with social (for want of a better word) issues, who would probably have easily bred using AI (if it were allowed for TB's).