The login script for a technical:
First validates that the key you entered is valid for your account.

Second it ensures that the key you provided is the Posting key, not another key, if you try to login using your master key it will reject it.

Third it checks if you are already a member, if you aren't a member it creates your account and logs you in.

If you are a member, it checks to see if the posting key provided matches the hash we store, if they do not match, we update your private key with the new one you entered that is currently valid, then you are logged in.

This all happens at once and should be transparent to the average user.