What does it mean when we say a dog is "over threshold?" It means the dog is over the optimal level of arousal to learn or perform. As with all terminology, the exact meaning will vary according to the culture you are speaking within. A person in the protection sports talking about over threshold may have a different tolerance level than a person who specializes in changing dog behavior.   But at the end of the day, we say a dog is over threshold when it is no longer able to perform or learn. It would probably be more accurate to say no longer able to perform or learn at an "optimal level", because even dogs who are extremely over threshold sometimes manage to perform or learn, especially if their life is on the line. But extremes of "feelings" tends to require extreme interventions, and most of us can agree that learning is slow and hard to accomplish when extreme interventions are required to get the learner's attention, and performing tends to lack accuracy under similar circumstances. Over threshold can be driven by fear: If a lion is standing behind you and you're pretty sure he's hungry, most of your energy is going to be spent trying not to panic and very little will be available to focus on the conversations taking place around you - maybe conversation from another person who is inexplicably unable to see the lion themselves.  Presumably some dogs feel that way when they're at a dog class and they see every dog as a threat, even if the owner doesn't, and the owner just wants the dog to learn or perform a set of skills. Over threshold can be driven by excitement and desire: A handler holding an amazing piece of food and a dog who hasn't eaten in two days.  The dog is so fixated on that food (and the possibility of getting it) that his brain can't stop long enough to hear the cues being given by the handler; the same cues that would actually get the dog that cookie.  Or maybe the dog heard the cues but the handler is trying to get three minutes worth of work for that amazing food and the dog doesn't understand that.  He just knows that for two minutes he worked his absolute hardest and he's not getting anything, so the combination of frustration and panic (over the feeling of possibly starving) and split focus between trying to follow cues and thinking about that food...it's too much.  The dog is over threshold and unable to perform. Dogs can perform over threshold for much longer than they can learn because performing can come down to habit and muscle memory but learning requires concentration.  The more the trained performance is one of rote execution (retrieve over high jump) vs. paying attention (cue discrimination), the more the issue of threshold will become a significant factor. The goal is to find the correct level of arousal for learning and training.  For most dogs the correct level will be lower for learning than for rehearsing which is why we often use lower value motivators when teaching than when performing.  Your goal should be the "optimal" level of arousal - "over" threshold suggests inhibited performing or learning because the dog is too stressed or excited (a form of stress) and "under" threshold means not motivated to perform or learn, though I don't normally hear people say that.  Then they usually say they dog is unmotivated or low drive.