Firstly, there is never a guarantee of 100% safety around horses. It's a matter of optimising safety.
Horses have a blind spot behind them. They cannot see anyone approaching them from directly behind. It's always best to approach a horse from the front, where they can clearly see you. It's always good to talk to them as well, so you can be better assured of gaining their attention.
I have a mare who was eating, I was standing in front of her. I reached to pat her, she nearly jumped like a cat, she was so shocked. She was so absorbed in eating, she didn't see me reach to pet her.
You know you have their attention when they look at you, particularly if they turn to look at you. while they are looking at you, you have at least, part of their attention. If they are looking at you with both ears forward, you have their full attention in that instance. It's always possible to be distracted.
When approaching a horse and wanting to go to their rears, I've always taught my children to approach from the front and greet the horse. Have a pet and a little chat and catch up (which basically means pats and kisses).
Then as I walk down the horse towards the rear, I keep my hand on the horse's body and talk to the horse as I approach her rear. This way the horse can feel my progress as I walk towards the rear and she listens to me. She knows what to expect, as the rhythm is smooth and my touch is constant.
When I reach the back I'm careful to stand to one side of the rump and keep one hand placed on the horse. I can then use the other hand to inspect what I need to inspect at the rear of the horse. This is the assumption I'm at the back to check something.
If I need to lift the tail, I follow this procedure and keep my hand that I've trailed along the body on the horse and then touch the horse with my other hand and gently lift the tail. I try to warn the horse what I'm intending to do, by placing the hand on a neutral area, like the rump and then lead down to the tail. As the tail can be a fussy point for some horses.
If I need to walk behind to horse, I keep my hand that has trailed along the length of the body on the horses rump while I walk behind the horse, close to the rump, so she can feel me moving behind her. I talk to her while I do this.
To my way of thinking, it's about communication. I'm letting the horse know, as practically as possible what to expect from me.
Now when I came up behind my horse, where there were two other horses near her and she was searching for some food to nibble on. That is a situation where she is focused on finding the treats she is finishing on cleaning up and ensuring the other horses do not try and push her off her feed.
When she feels a tickle on the inside of her back leg, she assumes it's another horse moving in on her nuzzling at her and her instinct is to kick. I couldn't have broken all my safe guards in more way possible, except perhaps to bow down and place my head in range of her striking hoof. It was a fool hardy way to approach a horse and dangerous.
Afterwards she came up to me, she didn't like kicking me. I was in agony and managed to stroke her quickly as I hobbled out of the paddock and later went to the hospital for xrays.
The three key issues here are communication, focus and environment.
The handler needs to communicate to the horse what their intentions are (as best as possible)
The handler need to be focused on the task at hand. This way if the horse is distracted, the handler can be aware that the cues given to the horse may not be received.
What is within the horse's environment at that moment? Is the environment in a state of flux with other horses moving around? Dogs? Other people? Is the environment calm and static?
It's easy to take these for granted as we have a familiarity with our horses or our attention is distracted from the task at hand or we rush through our activities in our busy lives.
One rule I have managed to follow thus far:
I never walk behind a horse I do not know.
Following picture gallery of unsuspecting horse:
A picture of a horse giving it's complete attention to a situation
The correct end of a horse to approach
Do not approach from this end!
Distracted horse - asking for lift away from owner!