The Gentle Leader is a two-part collar. Part 1 is a regular sort of collar, fairly narrow, which fits right behind the ears. Part2 is connected to Part 1 under the chin. It is a simple loop which goes over the muzzle. It has an adjusting lock on the left hand side of the face and a ring for the lead on the right hand side. You can make it tighter or looser as needed, with the adjustment lock.
For pulling dogs, like Gypsy

, this eliminates the pulling. They don't want that pressure on their muzzles, which they tolerate on their muscular throats.
Does that help?
Frau