If someone puts a blade to your heart, you're going to have a rough time. They leave it there, for a long time. They aren't going to cut you, but you can't move. You have to endure the pain and conceal your discomfort with the situation.

Learning to endure and conceal is all part of your ninja (にん) training. You must learn to stay totally still, without breathing — you have to be able to endure this if you want to conceal yourself.