A tulpa is an "imaginary friend" (a vivid hallucination of an external consciousness) created through intense prolonged visualization/practice (about an hour a day for two months). People who claim to have created tulpas say that the hallucination looks and sounds realistic. Some claim that the tulpa can remember things they've consciously forgotten or is better than them at mental math.

Here's an FAQ, a list of guides and a subreddit.

Not sure whether this is actually possible (I'd guess it would be basically impossible for the 3% of people who are incapable of mental imagery, for instance); many people on the subreddit are unreliable, such as occult enthusiasts (who believe in magick and think that tulpas are more than just hallucinations) and 13-year-old boys.

If this is real, there's probably some way of using this to develop skills faster or become more productive.