つい describes an action done inadvertently, accidentally, or by reflex because one couldn't help themselves. It originally suggested being right up against something, implying an immediate or impulsive action.
That is how Fuguro works. It brings つい back for review right before you forget it, each time a little later, until it sticks for good. You hear it spoken aloud, and you meet it again in stories you can already read. The first five levels are free.