A Martingale collar is like a normal flat collar but it tightens just enough that the dog can't slip out of it.
I like to use a martingale as a normal collar instead of a flat collar most of the time. Just to ensure the dog can't slip out of it. We like these collars.
Just like all other collars we do not recommend that you just let the dog pull against it.
We also never recommend that any collar is left on any dog unsupervised. In a crate, house, backyard or any other area unsupervised. But PLEASE Microchip your dogs!