[CentOS] authorized_keys command=""

Hello,

as I have read manual, if I use in file authorized_keys option
command="" with some command, no other commands will be permitted. I
have tried it, created authorized_keys2 for root and added there
command="rdiff-backup --server" and after that tried to login. Thit
command was executed, but I was normally able to supply other comand
as root. Can you tell me why?

Thank you
Martin