When a fast bowler is running into bowl should the batter be watching the bowler run in and then focusing on their hand and ball as they release the ball or should they be watching the ball in the bowlers hand the whole time they are running in as well?

I like switching off between deliveries when the ball is dead to conserve mental energy. My approach is to lock my eyes on the ball in the bowler's hand when they are at the top of their run-up and focus on it as they run in to deliver the ball. When I am doing this well, I tend to score runs more often than not.

I generally watch the bowler turn not intense , however when close to stumps switch on to intense concentration.
I think it’s too hard to concentrate for more than a few seconds .