What is the name of the optical illusion technique Dalí developed?

The optical illusion technique Dalí developed was the paranoiac-critical method.

Dalí developed the method in the early 1930s as a Surrealist approach to making images from irrational associations. He was interested in paranoia’s ability to make the mind perceive meaningful connections between objects that are not rationally linked. Rather than passively recording a dream, the artist actively interpreted ambiguous forms and incorporated multiple possible readings into the artwork.

The method is especially associated with Dalí’s double and multiple images, in which one arrangement can be seen as two different subjects. Reflections, landscapes, and ordinary objects could therefore shift into faces, animals, or architectural forms depending on how the viewer looked. Dalí discussed the method in his 1935 essay The Conquest of the Irrational, and André Breton praised it as an important Surrealist instrument. It is often confused with Surrealist automatism, but automatism emphasized uncontrolled creation, whereas Dalí’s method combined irrational perception with deliberate, critical construction.