noun - Ishin denshin (or 以心伝心) is a Japanese concept of communication. Out in the ocean, during a lake swim or sitting on an escort boat, many experienced open water swimming coaches and athletes employ this form of communication.
Because the means of communication is limited during marathon swims, many times information between coach and athlete must be conveyed in unspoken or unwritten ways. This nonverbal, mutual understanding between a coach and an open water swimmer is something special and must be cultivated over time.
When open water swimmers and their coaches communicate through a nod, a smile, a thumbs up, a wink, a wave or a "look" (seen through a pair of goggles or sunglasses) while in the ocean, lake, sea, bay or river, this ishin denshin just feels right. It communicates what is necessary. Both the coach on board an escort boat or kayak and the athlete in the water understand the communication and internalize the information being relayed from coach-to-athlete and from athlete-to-coach.
These implied communication skills, that may be unfathomable by others onshore or on the escort boat, are one reason why many people in the open water world understand the concept of 'what the mind thinks the heart transmits.'