To me focus means to not move the eyes but to bring the attention in that area and be aware of the sensations there. For example, just bring your attention on your left foot big toe at this moment and suddenly you are now aware of your big toe which was not in your awareness otherwise. Just keep your awareness here and you're focusing on it.
There is a important point to be made here; viz. The physical activity leads the mental focus/attention. In your example, flexing and relaxing the left foot big toe makes it far easier to bring the focus initially on to the activity/sensation at that point and then expanding it to overall awareness.
In the exercise i mentioned, you physically focus the eyes at a point in the blackness and the mental focus/attention follows it simultaneously.
