I don't own this (yet) but tried out my friend's.
It's a small harness that you wear and your lens attaches on the front of your chest, pointing down at your feet.
Extremely convenient for wildlife shooting where you are walking through a forest for a bit before encountering a subject, and need to quickly and easily grab your camera.
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A nice bonus is they sell a model that holds your binoculars too. Too often I leave my binoculars in my bag because it's too clumsy to use both a long lens and binocs at the same time (or you could put a smaller camera there).
I tried on my friend's using his 500mm lens and it was very comfortable. I really like how it distributes the weight to your shoulders, as a backpack does, so you don't feel the stress of all the heavy gear in one place.
There is a safety backup in case your lens were to come out of the harness, it won't be dropped.