Yesterday I was riding my green 6yo in a cross country lesson with trainer Andrew Lovell. We were chatting about my horse who is very easily sidetracked by what is going on around him while he is supposed to be focused on the fence ahead.
Andrew said that if your horse loses focus (fence judges, other fences, etc) and you need to get their attention back to you that you should shimmy your bum because it makes the horse wonder what is happening, brings the attention back to you while also making the hind end come beneath them all in a nice positive way.
This technique also works well for horses who lose concentration in walk work during dressage tests. In this case you would shimmy while sat in the saddle.