Half fill the toy with either cut apples/carrots or dried horse pellets and hang above the stall.
The horse should just reach the plait hanging down.
When the horse pulls down, the ball will flip up causing the food to be released and also to rattle.
All horses are pretty smart and driven by the smell of food, they will soon work out that by pulling on the plait in the attempt to gain the food, the food will be released!
The noise is also a stimulus which will entertain the animal long after the food is gone.