"'The one with whom you find your gods will not live' -- and from that curse, Rochel died along the way (Rashi, in the name of the Midrash)."
Chazal state that a curse that issues forth from the mouth of a sage will not fall flat; it will not be without effect.
Even if it is said conditionally, to begin with, and that condition is not realized -- it will still have an impact.
The list of examples related in Tanach is a long one.
One involves the warning Eli gave to Shmuel not to withhold the difficult prophecy he had heard regarding him -- that his children would die in his lifetime. If he did, the sentence would revert to Shmuel's own `head.' Shmuel did not conceal it; he revealed the prophetic decree he had heard to Eli, but nonetheless, due to this Shmuel suffered: his sons turned out to be improper (Rashi, Shmuel 3).
It was the same when underground waters were gushing up and could not be stemmed, thus threatening the world with another flood. Dovid warned that whoever had wise council and did not reveal it, would die of suffocation. Achitofel heard it and told Dovid what to do. Years later, we learn that he gave his last will and testament -- and died from choking. From here we learn that the curse of a sage comes true, even if the conditions do not apply, that is, even if everything was done according to the sage's own request (Makkos 11)!
This is also what happened to Yehuda, who declared, "If I do not bring him back to you, I will have sinned -- I will be ostracized." But he did bring Binyomin back alive.
Nevertheless, his bones were subject to this niddui for a long time (see: Bovo Kama 92 and Rashi there).
And the list goes on.
The question arises why this is so? From where is derived the tremendous latent power inherent in the speech that emits from the mouth of a Torah sage? A power that strikes and hits, come what may.
This question, however, stems from a double error. A misjudgment in the estimation of the power of speech, and an error in estimating the power of a single word.
In mortal eyes, only something tangible seems to have substance and reality. The spiritual, we mistakenly think, is something relegated to the heart.
The truth is quite the opposite. Spirituality is that which is real and lasting, while the material and physical is only a partial reflection of it, a proxy, as it were. No more.
"With the word of Hashem were the heavens made." The word of Hashem effected and produced the creation of the world. He spoke -- and it came into being. When man received the living spirit from Hashem, the soul which is a very part of the Divine, this was also reflected in speech. The "spirit of speech" is synonymous with "soul." And the soul's power of speech is also potent enough to create worlds. "And I put My words in your mouth to lay out the heavens and to establish the earth." "For with his deeds and his speech and his thoughts he maintains and gives power through many forms of power and elevated worlds" (Nefesh HaChaim Gate One).
The speech of the soul has effective power similar to that of the Master of all souls.
This holds true only when speech is a tool of expression for the soul. A mouth that is opened wide, that is used frequently, indiscriminately, gradually loses its connection to the soul. The tongue stops serving as "the quill of the heart," as it is described by Chovos Halevovos. The soul is not equipped to deal with vanities, inanities, empty words that fill that mouth. These are not an expression of the soul. They are vapid nothingness.
"The tongue of the wise is healing." The tongue of sages is, indeed, a pen of the soul, its tool and means of expression.
The wise are silent, and keep silent. When they do speak up, it is their soul that speaks from their throats. And when the soul speaks, its words have limitless power. The words of the soul can kill. The curse of a sage is volatile. Once expressed, it has become reality and cannot be retracted, even if the conditions that modified it no longer hold true.
The words have been given a life of their own, as it were.
The image of G-d (man) has spoken -- and it came into being.
It commanded -- and that became standing.
When speech is of this kind, isolated words are sufficient.
The silence that precedes the words draws them taut as an arrow flexed against a bowstring. The deeper the preceding silence, the more recoil and greater impact it will have when it is released. It will have so much stored energy, that its triggering will be like an atom bomb.
This can be proven in a different area, not that of a curse.
What countless words of an ordinary person cannot effect, the single word of a great man can. A brief sentence of a great man sometimes has a far more potent impact than lengthy orations. This can be testified by those dealing with the tshuva phenomenon, and the returners themselves.
Actually, any person will agree this is so. Anyone who has ever been struck by a flash of understanding following a brief utterance of a great man, a person whose soul speaks through his mouthpiece, will admit that it was the Shechina speaking from his throat.
Rabbenu Yeruchom of Mir zt'l often repeats the message that spirituality is a simple, basic reality. "And the sons struggled together within her." Whenever Rivka passed a place of idolatry, Esav would struggle to emerge, and when she passed houses of study, Yaakov would struggle to emerge, explain Chazal.
R' Yeruchom writes that a law of magnetism applies in spiritual things. Holiness constitutes an attracting pole, just like the needle of a compass always pointing to the north. The power that propels it is invisible, but it operates, nonetheless. And similarly for evil, which attracts like a magnet.
By a person whose every single word is a spiritual entity, a reality, these words assume power and potency, accordingly.
It is ridiculous how some people have so debased the sanctity of speech, in advertising for example, and they claim that "Words can kill." This happens to be the truth. But it is a truth that does not apply to the likes of people whose speech reflects a base animal nature.
Words, given the proper dimension, can do everything.
Primarily, to give life!