What does a martingale dog training collar do?

I have only just heard about them and was interested to know if they help to train a dog to walk on a lead properly without pulling or barking. Thanks if you can help.
Answer:
They can be used instead of a chain (collar). The fabric is easier on fragile necks than the regular chain collar. when adjusted properly, it will not close around the neck like a chain collar But you still get the chain sound of the correction. I do not reccommend a flat, chain or martingale for a dog that has pulling issues. For that a prong collar used correctly is actually much safer for the trachea as it stops the pulling with the least pressure on the trachea.

I use the martingale for my two poms. Poms have delicate neck and also they have large heads compared to their necks and a regular chain collar is too long when it is long enough to get over their heads. The martingale can be adjusted after it is put on so the chain does not hang down too low. They do not have pulling issues but a quick jiggle of the chain will put them back into proper position. Plain flat collars are too damaging for small dog's necks when training or the dog is pulling. They have no "give" and no warning "jiggle".