Similar to detection training. If we always provide the reward at the source of the odor - the dog will stay at the source of the odor. It is not about fooling the dog into thinking that the reward comes from the source. Dogs perform a great many behaviors knowing that the reward comes from the handler. However if he learns to expect the reward at the source of odor that is where he is going to stay. A detection dog who wont leave the source of the odor ~ what a problem to have!
In addition to proper timing, it's important to remember:
Dogs will learn to go to the place where their reward will be received.