The tertiary function is a stress-relief function, which means for INFPs, Si use may be a matter of preferring the familiar as an emotional comfort (when otherwise they'd prefer novelty), and focusing on detailed, sensory, almost reflexive hobbies to relax (such as creating detailed drawings or playing an instrument). They got lost in comforting, familiar, sensory activity or information reviewing, basically. This can go bad when they get stuck on reviewing negative past experience. Since Si is very idiosyncratic in nature, it can add a level of eccentricity also, the very opposite of the "earthy, grounded SJ" when appearing in an INP.

Te usage means growth becomes it kicks Ne into gear, so to speak.