how dose a Neck Holster work
I have never heard of that I just have mine on a pocket
Cord/chain goes around your neck and then threads through holes in the bottom of the sheath. Knife hangs blade up in the sheath under your shirt. You reach up from the bottom and pull it out. Almost all neck knives are small fixed blades, with a blade length less than 4". Almost all sheaths retain their knives via friction, but a VERY small percentage use magnets (e.g. Bud Nealy Escort).
As with everything, there's an exception to every rule: I have a neck knife sheath for a Spyderco Endura Wave that uses the wave to pop the knife open as it's drawn from the sheath. It came on a ball chain. My Spyderco Ronin has a channel at the bottom of the sheath for the chain to pass through. This is a flatter solution. It has a ball chain inside a shoelace for the chain. The ball chain will break in an emergency, which is why people like it, and the shoelace sheath makes it black and harder to see as well as more comfortable.