Isshin (one-heart) means to throw oneself wholly into the action without any other thought at all. Zanshin (remaining-heart) means some awareness still remaining. Some of the texts give both, some of them do not mention zanshin at all, and some of them mention it but say that the heading ‘zanshin’ means that there must be no zanshin.
With a spear, isshin is to commit one’s body wholly to the thrust; in a judo throw, it means to throw one’s body and heart at the opponent. If the action is technically defective, or the opponent more skilful, it will miss; then one is generally in an unfavourable position. On the other hand, the mere impetuosity and immediacy and completeness of the movement may have so upset him that he cannot utilize his momentary advantage.
Still, in theory might it not be better to take into account possible failure, and keep something back in order to be able to adapt to it? But then, the ‘one-heart’ will be broken into two: one saying ‘everything into the throw’ and the other ‘what if it fails?’ The latter is called a fox-doubt, and it infects the physical movement, to cause hesitation.
Similarly, if the attack is successful, must a new isshin be formed to deal with a new opponent?
The schools which speak of zanshin take it as an awareness which is wide and unmoving, and which contains the isshin. The immediate awareness is thrown into the action, and yet something remains, unconscious? conscious?, which can handle a failure or even a success. This is zanshin. It must not be consciously aimed at, as that would split the ‘one-heart’.
In a sense the one-heart is the ji or particular technique, and the remaining-heart is the ri or universal principle which manifests in particular situations but is not exhausted in them.
Isshin is the unity of the wave, zanshin is unity of the water. Isshin defeats an opponent at a time and place, by a technique. Zanshin is awareness of the whole process of defeating opponents, and wider than that, defeating them with minimum harm to them, and wider than that, for a good purpose, and wider than that. . . .
In the verses, zanshin is referred to as ‘water holding the moon’.