You say to your leader, “Believe and trust in me, I’m responsible.” The leader doesn’t think you can handle your duties, so this is why you said this to him. You want him to believe and trust you. You can handle this task.
To show you are serious as you say, “Believe and trust in me, I’m responsible,” you kneel down on each shin (しん). You have shin splints, so it hurts a bit, but you do it anyway to show how serious you are.