The humane twitch are metal or wooden pliers consisting of two levers, which are connected to each other by means of a hinge, a ring or a rope. The two arms are straight or bent. In the former case, the joint ensures that there is space left when the tool is squeezed shut; so the lip is not squashed. The double arc in the second case serves the same purpose. A rack on one of the arms and a ring on the other make it possible to hold the tool in a certain position; a rope attached to an arm fulfills the same role.
The humane twitch pinches the very sensitive upper lip (1) of a horse during treatment to divert the animal's attention and to induce a calming and sedative effect in case of disturbing (e.g. eye care) or painful procedures (cf. nose twitch) (2). [MOT]
(1) According to '' Antique medical instruments '': 226 the tongue would be caught, but there seem to be no other traces of that method.
(2) From an animal welfare point of view, the nose twitch seems to be preferred...