It usually involves a chest harness or issue load carrying gear with shoulder straps. Most frequently duct tape or large 1 1/2 rubber bands secure it to the strap, edge out away from the neck. The sheath must have high security to keep snags from opening the keeper straps and dropping the blade when you least expect it.
I don't know what gung-ho guy started this in the military, I've tried it with Glock knives and others. It can be done, but after two weeks in the field, I had to ask what was the problem it addressed, and did it solve it without creating others. I gave it a closer survey among more highly trained pro's and saw they shunned the practice, and in fact, didn't carry big blades all that often.