When the dog scents game it might point, hopefully, or it might charge to flush. Either way, the handler or dog flushes the bird and the dog chases. The handler hauls back hard on the line, the dog spins around the air, falls with a crack on his back and is quickly stacked back up back where the bird was with another WHOA bellowed out. The clever dog after doing this a dozen times on a 30 ft. line runs out 29 feet, does a neat back flip, darts back to where the bird was and stands staunchly, smirking while the bird flies away. The normal dog, which thankfully most are, gradually figures out to chase is to get hurt and so stops chasing a flushed bird.
All reinforcement is negative but is effective in breaking the dog to wing.
The same process is used to steady the dog to shot. In today's world of high tech dog training we can use the chock collar instead of the check cord. The concept is the same, only the effective distance is different. The learning involved in breaking follows the classical condition sequence. All of this training is dependent on the dog being in full chase.