Every time a PR boards a flight to Canada, his/her PR card is scanned by the check-in agent and added to the record. Just like a passport is.

There is now an IT link between airline systems and the CBSA, producing a "go" or "no-go" flag.

If you are a PR but have in the past travelled without your PR card, you can no longer do so. Just like a dual citizen who has in the past returned to Canada using a foreign passport from a visa-free country (say Britain). You now need to show your Canadian passport.

And that's it.