Short answer: replace this with
retain manual 3
rsnapshot manual whenever you'd like to do a backup.
The default configuration defines three different “backup levels.” The idea is that you run
rsnapshot hourly every few hours,
rsnapshot weekly every week, and
rsnapshot monthly every month (usually from cron jobs). When
rsnapshot weekly gets run, it moves the oldest
hourly backup to a
weekly backup; when
rsnapshot monthly gets run, it moves the oldest
weekly backup to a
The names for these backup levels, however, are totally arbitrary – there's nothing about rsnapshot that requires you to back up hourly, weekly, and daily. So if you just replace the backup levels with one of your own – a
manual level – you can just run
rsnapshot manual whenever you want, and it'll do a full incremental backup of whatever you've selected in rsnapshot.conf.
There's a lot more information about this in the configuration section of rsnapshot(1) – search for “retain”.