I think I have a good idea of what you're talking about, same as some others, blurring my vision does not require much effort, but comes as a natural instinct or feeling to do so. Just like you not being taught how to move your arms, you just do. This ability has been quite useless but I think it is quite interesting regarding there isn't much information on it.
(I have a feeling this has a correlation to the finger trick, where you move it close and focus on it, just without a finger.)