What word do you see here?
There are actually two words: “victory” in white, and “defeat in fusia. There are a lot of fun examples of this. Here is another good one.
It spells out both “optical” and “illusion”.
This is a verbal version of the multi-stable perception Gestalt effect seen in the Hidden Animals illusions. Ambiguous objects or drawings, like these, pop unstably between one interpretation and the other. Unfortunately, Gestalt theory only notes that this occurs, but does not provide neurological explanation for why it happens.