Today, while looking up something entirely different, I stumbled on the original patent for Dr. Cook's bridle, which contained this terrifying addendum: "The centerpiece may include a plurality of holes for receiving studs for applying painless pressure on regions of special acuity at the poll and behind each ear of the animal, or may receive a separate sleeve which includes the studs in order to apply pressure over areas of special acuity. Studs of different sizes can be fitted in a range of locations, depending upon the amount of pressure required and the conformation of any particular horse or other animal."
Thankfully, I have never seen poll studs in use at any of my barns, nor even seen them for sale.
Figures from Patent for
"Bitless bridle for governing horses and other animals"