I'm going off memory here, but there are transfer windows. You can't just enter the portal whenever you feel like it. There are two transfer windows per year. If you don't get in during the windows, too bad. They are not that big either, one is like 30-45 days and the other is like 2 weeks.
Also, if you sign and NLI you may have committed yourself for a certain time period, maybe one year.
I am not 100 percent on this, I am going off memory and don't have time to look up right now, but I do know for a fact there are transfer windows when you are allowed to enter the portal.