When you use right hand,
you should keep your mind in left hand.
I think this poem is including just same advice as Taijustu movement.
I always give this advice to my disciples. Also, this advice was given by my sensei to me before.
Regarding our body, when we attack, we keep mind in the part which is used for attack like fist and feet. We are careful to the parts contacting an enemy.
From my viewpoint, when the person who has that kind of sense doesn't have stable posture and smooth movement mostly.
I can find the center of his balance and the place of the heart even by a glance immediately.
It may be said that the partner that the center of the balance and the place of the heart do not become clear is a considerable expert. By my experience.
Both the mind and the body lose the balance surely if he put his mind in the part to use for attack. When we attempt to attack to expert, we may feel scare. I believe by this reason.
Maybe you can replace right hand and left hand with upper body and lower body in this poem. When you use upper body, you had better to keep your mind in lower body. Actually, there are still the part which replaceable, but please think by yourself.
First, you should keep mind at the part which balanced. Next, you can place and move your mind freely with good balance. You may delete your mind finally. I believe that is the reason why having ego is very bad for Bugeisha.
I was really surprised that such a traditional arts required good body posture and mind posture. I believe this thoughtful advice for balance and harmony makes wonderful Japanese traditional arts. It felt admiration for the culture of Japan that had mentally improved "Act of drinking tea" as a Japanese.