If 'Active' mode is choosen each sms is scanned. If 'Passive' mode is choosen only when there is a major change in the device state (like SIM card change, offline). Passive mode has to be tested more. I have jotted few FAQs in rs-faq.txt file. It should be installed in /usr/share/doc/rs directory.

The background of having two modes was the assumption that power could be saved. But, not sure if this assumption is really true.