For a HANS device to work properly, the shoulder harness needs to be properly mounted. The Owner's manual for the device is fairly clear, but some users of the HANS device appear to have not really paid enough attention. (there's a link to the owner's manual on the HANS website, it's a pdf but downloads "funny", you'll probably want to drop it on your desktop and rename it so that your pdf viewer will launch.)
The problem is that the shoulder harness mounting should be about 3" center to center, close behind the driver's shoulders. Additionally, the harness should drop 1" to 2" from the top of the shoulders to the mounting point. This is important because of the need to capture the "horse collar" of the HANS device with the shoulder harness; if the harness is not properly mounted, it may tend to slip off the HANS device and bad things may ensue.
Who should worry about this? Well, everyone with a HANS device, but in particular drivers of purpose built formula cars and sports racers should look long and hard at the factory provided mounting points. A great many of these cars have stock mounting points that are 8" to 10" apart, which are completely inappropriate for the HANS. I wouldn't use a HANS device in these cars until the mounting points got fixed.
This isn't really the fault of the makers of these cars; many (most?) of these designs were done before the HANS device became a big deal. But if you want to use a HANS in one of these cars, you need to mount the shoulder harness correctly.