The description of Zanshin can be started by firmly stating that it is difficult to describe it in words. It is a part of yuko datotsu 有効打突 and I think people focus on displaying zanshin more that kime. However, they are both needed. Zanshin is not straight forward; it is a complex concept that includes technical skill, mental composure, and physical presence. It is somewhat like the Tao(道) as described by Lao Tzu, any attempt to describe it in words does not do it justice. Much more experienced scholars have described the concept as well, one of the best being the TED talk (Zanshin – The Lingering Mind in Budo: at TEDxMeieki) by Dr. Alex Bennett.
That being said, I’m am still going to try a poor attempt to do just that. Zanshin is a state of mental and physical alertness, where one is aware of all that transpires around them yet not attached to any particular thing. In kendo, it is a state of mind and body, after striking with sutemi, in which one ready to respond to any actions performed by the opponent. It can be further defined as a display of full intent to attack and to correctly follow through until one has established the proper distance between oneself and the opponent and then turns and faces them again with full commitment of mind and body.
The power of zanshin is not like a tsunami, that is a single wave that carries with it a tremendous force just for an instant; rather it is more like the ocean itself, alive and filled with infinite potential energy. Upon inspection of the radicals that make up the character one firsts finds zan (残) meaning lingering or remaining. It is often understood as gradually extinguishing. This is paired with shin（心), readily thought of as heart or mind. Hence, zanshin is the spirit that lingers on. Thus, the spirit of all kenshi of the past are still present when we continue to train and preserve the teachings they have past on, linking us in an eternal chain.