Some commentary from WHINE:
With BPD, the messages that are sent are sometimes not in tune with the actual environment – there may indeed be no basis in reality for her reactions. An ancient Hindu text characterizes this “misperception” of reality in the following manner: “A rope may be momentarily perceived as a snake before ignorance is lifted.”11 The importance of this “ignorance” is that during the time the rope is perceived as a snake, your emotions react almost automatically. (I say “almost” because if you have been taught to love snakes and not to fear them, you will not have a fear reaction even if you misperceive the rope as a snake). You feel fear, it is real, and you jump away. Your body reacts as well. When I say “feel fear,” I really mean it. Your heart rate increases, the capillaries in your extremities contract to save blood for vital organs, adrenaline is released to your blood stream. Your fear is real and felt directly. However, it is based on a misperception of reality. When you see that it is actually a rope, you might feel foolish or you might, if you had BPD, still try to convince everyone else that it is really a snake even though others can see it is a rope. The reason for this behavior is that the feelings are so immediate and seem so “true” that you have to make “reality” match your feelings, rather than the other way around. When an emotional reaction conflicts with the state of the environment for whatever reason, it is said to be a “misaligned” emotional reaction.
The core problem with BPD is poor emotional regulation. That particular problem can cause other symptoms to arise as the person with BPD becomes emotionally dysregulated. This term emotionally dysregulated (or just dysregulated) is used to denote the state in which a person with BPD is overcome with powerful and, at many times, misaligned emotional reactions. Remember that emotions don’t arise on their own; they are based on cues or triggers from the environment and compared by our “emotional immune system” to the meaning of the cue. For a person with BPD, the meaning can be misjudged or, as is more often the case, the sensitivity to emotional cues is greatly heightened.
An example is a heat-sensing system that helps to detect and suppress fires. Sometimes companies will install heat-sensing equipment in addition to smoke detectors so that they can protect assets that need a certain temperature to operate (e.g. computer equipment which might cease working at a high temperature). The setting at which an alarm goes off might be 80 degrees Fahrenheit. In the case of someone with BPD, the setting (or “tolerance” as it is called in the control community) is naturally set much lower, at say, 50 degrees Fahrenheit. That means that the alarm will be raised much more often and lead to a reaction to the alarm. In other words, people with BPD will experience many, many (what you would consider) false alarms. However, these false alarms seem completely real to them, because their tolerance for emotional triggers is set very low. They are constantly running a fire drill. Unfortunately for you, the BP may drag you along unwillingly and unwittingly for the drill.
When Hope is Not Enough ebook (Kindle Locations 373-374).