Your ISP geneally has a specific range of IP's that are permitted for use in a specific area. when your IP changes you are still within this range. This is a fingerprint of where you live (often by city).

The idea behind the system Rift uses is that if someone outside your range (district or city) tries to log into your account, everything will be locked. If this happens, you will be sent an e-mail with an unlock code. If you are the one logging in and it locks due to this, then the code sent to you will validate the new IP range.

I tried this out once and the entire process took no more then 20 seconds.
Log in .. "coin-lock: enter code"... "you have mail!" ... #### ... "New IP Validated!"