New theory coming up although I am sure it has been said before. Horses cannot differentiate between sensations caused by different sources - they can't say, oh, that's caused by electricity, or static, or the end of a whip or a human hand for example, particularly if they receive a sensation from one thing which they can't see which coincides with the appearance of something else that they can see. Confused? Take for example a horse such as the one I saw on Monday and am going to see again on Saturday. He backed up when slightly spooked by a tractor and came into contact with an electric fence which zapped him. Since then, he has become worried about tractors even though we know it wasn't the tractor that zapped him - just an unfortunate coincidence although the two events were connected. He has become superstitious about tractors and now reinforces his own fear by turning his bottom into any tractor that approached him from behind, making it more likely that it will get very close to him as it goes by and frighten him even more - a sort of self fulfilling prophecy.
Same with Jenny and Splash. Jenny fell off Splash when she spooked at something and as she did so Splash jumped to the side and was stabbed by a piece of twig - something that reinforced her fear of things falling against her side rather than of twigs.
I think horses like to know what it is that has 'got' them and will look for something to 'blame'. I don't think they can see the end of a lunging whip that is about to hit them and instead start to look for what they can see, i.e., the movement of an arm swinging towards them. When a horse is afraid of that movement it tells me a lot.