crontab + rsync problem

[Luca Ferrari]

On Thursday 11 October 2007 Andrew Glen-Young's cat, walking on the keyboard, 
wrote:
> I personally use separate user accounts, unique ssh keys without
> passphrases and authorized_keys files with 'from' and 'command'
> patterns in them. This, however, might not suit you.

I didn't get this very weel, could you please explain me better? I have 
already separate accounts and ssh keys without passpharase, but I don't 
understan what you mean with "from" and"command".

Luca