What Will does is psychometry, in every way that matters— aka the (fictional) psychic power described by parapsychologists as the ability to detect and interpret the energy imprints left on the psychic energy fields of people, places or things and the surrounding events and emotional energies that’ve affected those imprints. Jack having other people clear the scene for Will removes any interference from their energies, Will closing his eyes to enter a meditative-seeming dissociative state allows him to visualize the timeline of events the energies of the scene relay to him, and the person who would leave the most accurate energy imprint of the killer’s actions, emotions and motivations would be the killer, so Will re-experiences the events of the crime from their perspective.

And while Will is obviously a highly intelligent individual who’s had extensive education on behavioral psychology and forensics, a lot of the things he says about the killer’s mindsets and the details of their lives are way too specific to be educated guesses or projections of his own darker desires, especially with the fact that he’s always correct. People like Jack, Chilton and Hannibal who work in relevant fields have an interest in him as a result, but they describe him with reaching vagueries and mostly disregard the how of it in the end. Of course psychology, science and logic have nothing on him when he has abilities that can defy all of that like the ones Hannibal and the Angel Maker show, too. Will is probably the most obvious example of magical realism in a show accented with it throughout. He’s a magic 8 ball filled with snark and angst and we love him for it