Many retrievers have a natural instinct to break their heel or steady position once a bird is in the air or a gun has been fired. Basically, the dog will break naturally if you let him. Not only is this a nuisance when you're hunting, but you're dog won't mark near as well when he's breaking.
Steadying can be a long process, but it's an important one if you want to give your dogs the tools to help you succeed in the field. Here's a training technique that will help keep your dog steady until you're ready for it to retrieve.