Rebbe Nachman of Breslov tells us in Likutey Moharan (Lesson #160) the following: “The pulse knocks and beats in a person. Sometimes, it beats in a person and prompts them to G-d’s service… and sometimes, it prompts a person to sin, G-d forbid.”
Continues the Rebbe, “This is because the pulse comes from the breath, and the breath comes from the air by means of speech. Depending upon speech, such is the nature of the pulse; for good or the opposite.”
In other words, when a person speaks, the breath they emit fills the air around them. Their speech thus becomes a portion of the air that other people breathe. If the words are good and holy, the other people breath “clean” air and this produces a positive pulse prompting them to desire service of Hashem.
If, on the other hand, the words are evil, the other person breathes “polluted” air and this produces a negative pulse, prompting them to desire sin (Likutey Tefillos I, #120).
It is also possible to understand this as referring to the speaker themself: They breath the air filled with the words of their own speech and are influenced accordingly. As Rebbe Nachman teaches elsewhere, evil speech has the ability to bring the speaker to sin. One must therefore be very careful with the words they use (Sichos HaRan #237).
A story is told that the Baal Shem Tov once took hold of a doctor’s wrist, felt his pulse and detailed the illness from which the doctor suffered. After acknowledging the validity of the diagnosis, the doctor attempted to read the Baal Shem Tov’s pulse. Finding it to be erratic, he thought for a while but could not define the illness. Once the doctor left, the Baal Shem Tov, paraphrasing Shiur HaShiurim (5:8), said, “My ‘illness’ is that I am sick with love of G-d. The doctor has no understanding of this love. That’s why he was unable to read my pulse properly.” It goes without saying that all the Baal Shem Tov’s words were holy. Thus, his pulse raced totally for Hashem.