Standing behind your horse, in their blind spot, requires that there is some mutual Trust, Confidence and Respect in place!
We've always been told not to 'go behind' the horse and for many people, it is an instant heart-racer when they go 'back there'! Years of conditioning have caused people to feel fear, which of course the horse can sense. And when a horse senses fear from his human, he doesn't necessarily think the human is scared of him (he doesn't think he's scary!), he could be wondering what is back there in his blind spot that has his human so terrified!
Fear will cause him to potentially move his feet, which means he will shift his hindquarters to see what's going on back there, and sometimes this movement can be sudden, which of course scares the human even more!
I spend lots of time at the rear end of horses. It's a demonstration of 2-way trust, confidence and respect. To prepare yourself and your horse for this (please do not do this without preparation and understanding) may take a little time, and your safety is always my #1 concern.
While to many of my readers, this may not be a big deal, as their horses are relaxed and comfortable and love bum scratches! But some horses (and you never know when you are going to meet one of those ones!) may associate you being at their bottom with veterinary examinations etc., not altogether pleasant!
Above, I am with Squeeky, who has been experiencing separation anxiety for many years (she still lives with her mum) - getting very animated when they are apart, even with a fence between them. In this session we are building mutual trust and I am establishing my leadership to where she looks to me, not her mum, for guidance and direction. Things are coming along really well after a couple of sessions, with her human now able to take her for a walk without mum, and all is confident and relaxed.
Trust is earned, for both members of the horse/human partnership. Taking the time it takes (to borrow from PP) will pay off Big Time - and remember, a prepared horse is a Safe Horse!