This communication can occur when both people involved are awake, or if one of them is asleep. When one of them is sleeping, the other can show a virtual image of where he is through the person's dream. Both users also have the ability to sense where the other is and somewhat read their emotions. If one person dies, the other will most likely die as well or sink into a vegetative state. An empathy link can only be abolished if the two users are face-to-face and they want it to be removed.
Grover Underwood opens an empathy link to his best friend Percy Jackson after being taken prisoner by Polyphemus, a Cyclops. Percy Jackson, Annabeth Chase, and Tyson go to Grover's rescue. While Percy sleeps, Grover is able to transmit images of his location to Percy's dreams and communicate with him.
At the end of the book, Grover offers to have the link removed, explaining that it could kill Percy if Grover died, but Percy convinces him to leave it.
Aphrodite confronts Percy about his true intentions of going on a quest to rescue Artemis, knowing it is really to save Annabeth. Percy lies to the other questers about the meeting, but Grover is able to read his emotions well enough to figure out what was said.
Luke Castellan tries to convince Thalia Grace to join Kronos' side, by getting her to summon and destroy the Ophiotaurus, which Percy had named "Bessie." Once Percy starts thinking about Bessie, he begins to draw the monster towards him, and Grover has to mentally contact Percy and tell him to stop thinking about Bessie.
Grover's disappearance makes all his friends distraught, but Leneus claims that the empathy link is unreliable for communicating with Grover Underwood. Later, Percy senses Grover when Nico di Angelo takes him to Central Park and uses the empathy link. Once Percy uses the link to get in touch with Grover, he snaps Grover out of a deep sleep (caused by Morpheus), by telepathically telling him to think about food.
When Percy arrives at Camp Jupiter, Don smells Percy's empathy link with Grover, a satyr, because the Roman counterpart of fauns are satyrs. Once Don picks up on the link, he tells Percy that he has an empathy link with a faun, but that it was being blocked, before the subject is quickly changed by Nico di Angelo.