If someone puts a blade to your heart, you're going to have a rough time. They leave it there, for a long long time. They aren't going to cut you, but you can't move. You have to endure the pain and uncomfortableness of the situation.
This is all a part of your ninja (にん) training. You must learn to stay totally still, without breathing. This is how they train you. If you can endure this you can endure anything, plus you can stay really still, which is good for ninjas.
Currently there are no similar looking kanji which exist in the WaniKani learning system