"And then something strange happens. I feel energy welling up in my core and rushing out through my limbs, fingertips tingling. Maddy must feel something too - a rush, a burst of energy - because her eyes snap into focus."
(Number Nine, Nine's Legacy)
"Nine places his thumb on Sam's forehead. Sam's hair blows upward and I can smell electricity in the air..."
(Number Four, The Power of Six)
Legacy Transference, or Miras, is a Legacy which allows the user to share their other Legacies with Loric and transfer Loric essence (in order to use Loric items) and presumably Legacies to humans for a small amount of time, approximately ten minutes.
Nine first discovers this Legacy when he accidentally transfers his Antigravity to Maddy which would have helped them escape the Mogadorians that had ambushed him in her apartment, had Maddy not betrayed him.
Nine again shows this Legacy in the Mogadorian mountain base when he transfers Loric essence to Sam, which allows Sam to use the red X-Ray Stone, where he would not have been able to normally. This Legacy is also used later to briefly give Four the power of Antigravity.
Nine transfers his Antigravity Legacy to Marina before the Capture the Flag game, so she could escape from John to win the game.