Star Wars has just provided a full explanation of the mysterious Force Heal power seen in Star Wars: The Rise of Skywalker - and it has unintentionally created a plot hole in The Mandalorian. It may not have been a critical success, but Star Wars: The Rise of Skywalker certainly expanded the franchise in terms of Jedi lore and new Force powers, especially through Rey's character.
One of the most intriguing new elements was a Force Heal power unlike anything seen in the franchise before. It allows a Force-user to transfer some of their precious life energy into another being, healing them and even potentially restoring their life. Rey first used this ability in the desert of Pasaana, when she healed a sand serpent that was threatening the heroes, and later in the film she healed Kylo Ren from what should have been a fatal injury. There are clear limits to this Force Heal, though, because at the end Ben Solo returned the favor and healed Rey - sacrificing himself so that she could live.
Lucasfilm Publishing has just released The Star Wars Book, with contributions from Pablo Hidalgo, Dan Zehr, and Cole Horton, and in one section it explores Force abilities. According to The Star Wars Book, this Force Heal power is one that can only be used if you are part of a Force Dyad.
"Those in a dyad possess another unusual ability. They can use the Force to heal a wounded person or creature, but this act also requires that the healer transfer some of their Force energy, which can temporarily weaken them. After their battle with Darth Sidious, Ben Solo sacrifices his life and all of his Force energy to bring Rey back from the dead."
Lucasfilm is essentially trying to tidy up after Star Wars: The Rise of Skywalker, using tie-ins to explore unanswered questions and resolve mysteries. Unfortunately, this particular attempt is a problematic one, because it forgets Force Heal was seen in The Mandalorian season 1 as well. The power was demonstrated by Baby Yoda, who was able to use it to heal Din Djarin when he had been badly injured. There is no evidence Baby Yoda is part of a Force Dyad, especially given these are supposed to be extremely rare, so one is unlikely to occur twice within the space of just a couple of decades.
In truth, this explanation doesn't fit with previous tie-ins, with Rae Carson's novelization of Star Wars: The Rise of Skywalker explaining Rey learned Force Heal when she was repairing Luke's lightsaber. According to the novelization, Rey had pored over the ancient Jedi texts, and there she had learned powers that had been forgotten in the mists of time, including Force Heal. She had used this to restore the fractured kyber crystal at the heart of the lightsaber, transferring her own life energy into it. But if the power is inextricably linked to a Force Dyad, then Rey should surely have also come to understand her strange connection to Kylo Ren.
This is the problem with depending on tie-in books and reference material to straighten things out. The Star Wars franchise is ever-expanding, and it's simply impossible to keep track of every detail. Sooner or later, contradictions will emerge - and, unfortunately, this is a good example.