Changing inputs to your nervous system can make instantaneous changes to your performance. One of the easiest things you can do, particularly when training, is to shift your eye position. The correct eye position will improve strength and flexibility in certain positions.
When trying the activity shown in the video, (shoulder extension) see which eye position improves your range of motion. About 80% of you will see improvement with eyes up, the others with eyes down.
Generally speaking, when your eyes are looking up, you promote strength and flexibility in extension movements and when they are down flexion movements are enhanced. again, around 20% of people will see the opposite (as is the case in the video). As Dr. Higgins said, the important thing to try is using your positions to drive increases in motion and strength. See what works best for you.