A good use the shutter control feature is for removing the motion blur from low-light (indoor) slow-motion shots. When shooting at 120fps or greater, the camera will naturally open to a 360 degree shutter if isn't daylight. This will make the slow-motion very smooth, but too dreamy for some action sequences. Using a 90 degree shutter will keep the motion blur to a minimum for these demanding shooting conditions.
Switching to LOCK will disable any auto-exposure behaviors in the camera. In most lighted situations, setting the camera to a fixed ISO 100 is best, offering the lowest sensor noise. But all is not perfect, as GoPro cameras are designed for durability with no moving parts, there no iris (aperature) control. Fixing the ISO to 100 and the shutter to a longer than typical for action sports (normal is around 1/1000th second) often there will be too much light due to the wide open F2.8 lens. For daylight shooting you will need to add neutral density filters, and a lot. Fortunately the ND32 needed is available for direct lens mounting from this Polar Pro filter set (unfortunately they don't sell the ND32 separately.) There are other options to adapt the GoPro lens mount to traditional filters like this from Snake River Prototyping, great for handling for those rare days when you need ND64. I've used them all with excellent results.
Manual exposure is here, stock up on you ND.