German Martingal

Anybody here in this forum uses a German Martingal for training purposes?

I did for a while, but I'm not a fan.

If I have a beginner rider on a safe horse, it helps to protect the horses back, and mouth.

But personally, it doesn't teach you how to feel a horses mouth, its a shortcut, and the horse leans to pull against it.

They can be useful. I have one and use it occasionally. The thing about them, like any training aid is not to become dependent on the tool.
It doesn't replace training, it aids in the process.

Here is a video by Larry Trocha that gives a simple explanation. The adjustment is pretty important.