Not to argue a point with barrelracer, but I don't think the knots really work on pressure points. Look at how much a halter moves around when it is in use, the knots don't hit a precise place on the face, they move all over the place.
To quote a halter maker (web link follows the quote): "Here at Cypress Lodge we don’t believe the story that the halter knots work on pressure points. The stimulation of pressure points is an exact science. Pressure point locations are well documented and named, so are the energy lines they stimulate and the internal organs they effect. So if any halter maker can provide this information, then explain how they guarantee that their halters stimulate these exact points on each and every horse, please feel free to email and we’ll say sorry. Until that happens, and don’t hold your breathe, we maintain that’s it a load of bull****. These halters work in much more benevolent manner, they simply reward the horse for yielding to minimum pressure applied by the trainer or rider. While on the subject of pressure point knots we do not use, therefore, do not make rope halters with extra knots over the noseband. We believe these knots do not work on pressure points but rather create points of pressure on the very sensitive nasal region of the horse. We are of the opinion that these knots cause unnecessary discomfort and/or pain therefore not meeting our requirement for humane training tools." Cypress Lodge :: Promoting Humane Training and Treatment of Horses