Baseball--the canine version, not the MLB--is the foundation for teaching your dog to handle, or take directional hand signals. The pattern is set up just like a baseball field (hence the name) with a pitcher's mound and home plate, as well as first, second, and third bases.
Through repetition the dog, which starts by sitting on the pitcher's mound facing home plate (where you stand) learns to take a directional hand signal to one of the three bases to retrieve a bumper. The act of successfully retrieving an object, combined with lavish verbal and physical praise, is reward for the dog and builds his trust in you that wherever you cast him there will be something to retrieve.
You start with identifying each base individually so as to simplify the task and set the dog up for success. After the dog has learned each base and begins to comprehend the game, you can have bumpers out at each base and can cast him to whichever you feel like.
As baseball becomes simplistic, you modify it by running the dog from home plate. You line him up with second base and send him for the retrieve, eventually being able to stop him with a whistle when he reaches the pitcher's mound and cast him to whichever base you want. There are numerous drills that come off of this basic exercise, but it is the single most useful handling drill you can teach a dog.
There are many issues that can arise when you first start, however. Dogs often don't understand the concept at first and become confused when you try sending them for a retrieve from even just a few feet away. Most of training issues (for just about any dog) can be fixed by simplifying. When it comes to baseball that usually means identifying the pile by tossing a bumper to it so the task and destination of the dog is clear to him, and/or removing other piles to reduce conflicting messages.
The above video, an entire episode of Dan Hosford's "The Working Man's Retriever," illustrates why you need a hunting dog that can handle and then teaches you how to get started. It features a dog that's learning the concept for the first time, so you get an actual feel for what your dog might do wrong (and it does make a couple of errors) and how to correct it (or not, as the case might be). Note how Hosford makes the beginning of the drill very clear to the dog by repeatedly identifying the pile and slowly moving away from the dog.