In the Zohar's introduction to Parshas Bechukosai, it says that the letter yud once waged war with the letters kaf and samech. The Ari z"l explained that the yud represents the soul, nefesh, the samech, the spirit, ruach and the samech one's life force, neshamah. Therefore, the word "kis", which contains all of letters of the combatants represent all these forces.

Desire to do a sin is often far worse than the performance of the sin itself. Lack of "kis" causes one's "Naron" (nefesh, ruach and neshamah) to be diminished and wanting. Our verse uses the strong language of 'command' because strength of the soul is a matter which requires extreme diligence. If one's body is hungry for sustenance it screams and cries out. However, one's Naron is unlike the person's physical body; it does not call out if it is deficient but remains hungry. Therefore great diligence is required to maintain the proper nutrition and health of the Naron.

A parable demonstrates this point. A man once bought a fat chicken on Sunday to eat on the Sabbath and he gave it to his wife for safekeeping. That Tuesday he bought a fat rooster as well which he also gave to his wife. When the wife took the rooster to put in the coop with the chicken, she noticed how emaciated the chicken she had placed there had become because she had forgotten to feed her. She told this to her husband and he warned her that this may also happen to the rooster. She soothed him by saying that the rooster will cry out and cackle if he is hungry and therefore she will be constantly aware of his needs and not forget to feed him. The chicken, on the other hand, did not call out for sustenance as would the rooster. Similarly do our bodies call out to be fed when they are hungry; our Naron, however, require much greater observance and diligence to assure its survival and growth.