Psionic Links are when other Psions attempt to help add to the power of another Psion trying to perform a Psionic action. They do not have to have the same power, so for instance someone with Psychometabolism could help a Telekinetic move a large object. The only exception to this is that a Reiki master would have to be touching the Psion he's trying to help.
Psionic Links act as an Assisting roll. The biggest problem for someone using a Psionic Link to help another Psion is that they receive all Psyche or Fortitude damage the main Psion takes, and if the main Psion is injured, they receive the exact same injury.