....realized my response under ostrich belonged here......

It's a "twitch".

A loop of leather or heavy twine/light rope attached to a length of heavy dowel (or a sawn off bit of broom handle).

The poor nag's top lip is caught in the loop and the whole device is rotated - as one does with a tourniquet. It must be very painful - the horse freezes and will do whatever you want.

stales