My only experiences with HYPP came from working for a Paint trainer as a groom. I was from Arabian & TB horse background, so I had never heard of HYPP nor did I know any of the signs and symptoms. She never told me that just about every horse in that barn was N/H at the very least. Nor did she, until one had fallen on me, tell me anything to look for, how to work with it, nada.
Stress, improper feed, just because can all be the triggers for an attack. I learned to look for fasiculation of the muscles, kind of like they twitch when flies land on them, but no flies in the area. I learned that when I saw that I needed to give an oral syringe of about 60cc of Karo Corn Syrup to the horse. The sugar reverses the improper flow of potassium in some cases and can stop the attack. I learned to hook a lead rope on the horse and unhook the cross ties. If the horse was still fasiculating after giving 2 syringes of Karo, I learned to get out of the way in case the horse went on to have seizures. Of course, I learned that from UNDER a horse who fell on top of me and seized.
After working there for several months, I learned that I'll never voluntarily have or handle an HYPP N/H or H/H horse again. In the case of the show horses I worked with, some had fairly frequent attacks, some more rare and I've been told that some N/H horses never show a sign. I've not ever seen that, or if I have, I didn't know the horse was N/H. I certainly would never ride one who has tested N/H and has had an attack.