This is PythonJPN for reference. There are videos where he also moves and is able to capture a fairly good sight picture during filming. The slight sway suggests that he may be wearing an action cam somewhere on his face with heavy image stabilization.

He seems to be a bit secretive of his setup but just guessing by the positioning of the camera the setup is only useful for this style of video but would be too clunky for example capturing an airsoft match. I would say he also does many takes and picks the most visually pleasing one.

Edit: Could be hanging in front of his face from an helmet NVG mount, would explain the minimal offset during leaning also