Once you get five strings, you become your darkest self. You can also become your darkest self earlier than that! Say, the 7-9 lash out physically result, or the MC just makes a move because it's appropriate in the fiction, or you have sex with a Mortal, or whatever.
While you're your Darkest Self, your dark power can still gain strings on you... but you should probably try to limit yourself in using bargains, because the only way to escape your Darkest Self is to get rid of all of those by fulfilling your patron's commands. Well, that, or sign up with something worse.
So generally, yeah, you go up until you hit 5, or you become your Darkest Self through some other means, and then you go (mostly, hopefully) down until you hit 0, at which point you escape your Darkest Self.
Now, here's a fun twist: if somebody from another Skin picks up "Soul Debt", then they go into the Darkest Self for their Skin when the Dark Patron gets five skins. So a werewolf, say, wolfs out if they deal too much with their demon. Also, they don't get the "get rid of strings by fulfilling demands" clause that the Infernal gets in their Darkest Self, which means the only way those strings will go away is if their Darkest Patron uses them according to the normal rules for an NPC using strings on a PC. Which is a kind of interesting dynamic.