Buschkuehl's team postulates that the n-back task improves working memory -- how many pieces of information subjects can keep in their head -- as well as the ability to control the brain's attention. Fluid intelligence tests require those types of thinking, and the training improved performance in these underlying skills.
In the experiment reported in PNAS, the researchers did not find the upper-limit for improvement, suggesting that more training could yield even better mental performance gains.
"We saw a linear increase in performance with increase in training time."