Like most things in Star Wars: The Last Jedi, Luke Skywalker’s death divided fans. Rather than go down in a blaze of glory, he instead projected his image across the galaxy in an attempt to distract Kylo Ren and the First Order just long enough to allow the Resistance to escape. I thought it was a smart move from Rian Johnson, with Luke recognizing that he has far more power as a symbol than as a physical person waving around a laser sword. But one of the major criticisms was that this Force projection power hadn’t been properly established in the Star Wars universe.
Well, it turns out those critics hadn’t been reading up on their EU Star Wars novels. As revealed in the Star Wars: The Rise of Skywalker – The Visual Dictionary, this power has indeed been used before. The 1996 novel Before the Storm by Michael P. Kube-McDowell, set in the wake of Return of the Jedi, revealed that this technique was mastered by a species known as the Fallanassi (also known as the Adepts of the White Current) and was contained within the Jedi texts.
As per the book, Luke abandoned his spirit in the Living Force and poured it into the Cosmic Force, allowing him to project himself across the galaxy. It’s a powerful technique, and Luke’s version (which involved being able to physically interact with his surroundings) surrendered his spirit to the Force completely and he thus departed from the mortal world. The technique’s explained in The Visual Dictionary, indicating that the Lucasfilm Story Group have brought it from the novel into the present canon.
So, there you have it. You may not like the explanation that the book provides, but this is what Lucasfilm is going with. It’s also nice that despite being non-canon, they’re still prepared to take ideas and concepts from the many Star Wars EU novels and games.