"Mizu no kokoro" is a
Japanese phrase which roughly translates to "mind like water". It is a concept taught in
karate (and perhaps other
martial arts). If a
karateka has a mind like water, it means s/he looks at everything the same, regardless of what physical or mental state s/he may be in.
For instance, if you were in combat and suffered a severe hit, it would be best to give the impression that it didn't hurt you. Your opponent would be discouraged that his attack was ineffective, therefore giving you a psychological advantage.
If a karateka had reached a state of zanshin, mizu no kokoro would be effortless.