You need at least 70 pounds of pressure on the cord for it to pull out and inflate the vest.
Over the weekend I got off a 17.1 hh horse and forgot to unhook and it didn't deploy, but that's because it didn't have enough pressure put on it, and I also realized halfway off so I was on my tip toes as I unhooked.
Regardless, it's definitely something I'm saving up to buy. My coach swears by it and I know it's saved her from many a serious injury, as well as other riders. As I continue moving up the levels I will make sure I never ride cross country without one.