The catch is that once you log in, you have 96 hours of remaining free logins, so you can’t login freely over the whole period. But the fact that you can log in freely is the point.
Anyone who has registered the game and had an inactive account for 30 days or more is eligible to take part, although those whose accounts are inactive due to bans or suspension will not be able to overturn those as part of the free logins. Once you’re in, you can just play the game normally during the trial period. If you need a refresher or you’re thinking about giving it another shot, you can download the client and be ready to go.