This only works if you know what you are doing. I have seen several well-behaved, well-"broke" horses develop some really bad behaviors from being in the hands of somebody who thought they knew what they were doing, but really didn't. The problem is that a horse's training is a lifelong process; he's learning all the time. He may do something subtle that the handler doesn't notice (or thinks is cute), and he get away with it, so the next time he goes a bit further and gets away with that. Before you know it, he's a handful, and the owner says, "what happened? He used to be so sweet! I guess he just wasn't trained right," when what has happened is that they "trained" these new behaviors without realizing it.