Switching from Ortho Tri to NuvaRing

When does the nuvaring start to work and protect from pregnancy? I have been on ortho tri cyclin low for many years and i have decided to switch to nuvaring. I have been on the pill up until my menstrual cycle started on Sep 16th,(my placebo dark green pills). I inserted the nuvaring on the 18th and my period stopped. My doctor told me to take the one i have in now out on Oct 1st and insert a new one, then out on Oct 25th. I understand that he is getting me into a normal cycle (in on the 1st out on the 25th-repeat) but then when does the ring start protecting me from pregnancy?