I think that Jack is already aware of his love for Ennis, that don't surprise me in a way that he's holds Ennis's wrist, the fact that Ennis doesn't take is hand away more surprises me in this scene, could it be a way of accepting this ? Could he more or less understands what is really going on here despite of the hurry of the situation ? I don't know, you're right, at first glance FNIT seems so rough, that's why we need to watch the movie again and again, just to catch those precious moments !