I differ with the earlier posts. Practices like these are very meticulously designed and rules of a perfect practice are hard and fast. However, the approach, how to reach the perfect state, is different. One has to slowly and gently evolve to perfection. So, on the way, teachers advice little relaxation here or there, which does not mean that the practice is less precise or rigorous. The rigor of Yoga is in patience, in steadfast devotion and rising through the rungs of diluted efforts to a perfect state.
First, focusing on certain parts of the body (nose in this case) itself is a result of mind's energy, not the other way around. Senses are working anyways, what one has to consciously do is to become aware of it. Hence, moving the mind's attention there.
Secondly, looking cross-eyed (however, not forcing it) is required, natural and intended. Eyes are lenses hardwired to certain areas of the brain for the signal-procesing. That little strain on the eyes rejuvenates the whole sensing process.
Thirdly, the real intention is to build ability of sensing subtle things. So, by providing energy to the highly sensitive nerve centers in the tip of the nose, one needs to learn sensing the subtle air moving in and out. The otherwise ignored aspects like warmth, humidity, volume, direction, rate and consistency of flow of air needs to become alive.
Lastly, when this is done so comfortably that there is no strain, one would intuitively sense the chakras like sahasrara, bindu and ajna. And that is the real goal.