What exactly does sudo -su

Question

I have a linux user (user2) which has set as shell /usr/sbin/nologin (to block logins). No when I try to switch to this user with sudo su user2 I get the info that this account is not available. But whe I user sudo -su user it works fine.

What is now the difference between su and -su. Or what does -su. I tried to get the help with -h or --help or the manpage but I coulnd not find anything helpfull.