Short answer: replace this with
retain manual 3
and run 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 monthly
backup.
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.