The interweb has the details:

"If we don't meet our 20 minute guarantee, simply see an Alaska Airlines Customer Service Agent within 2 hours of your flight's arrival for your voucher."

and

"If your baggage is not at baggage claim within 20 minutes of your plane parking at the gate, you're entitled to a $20 Discount Code for use on a future Alaska Airlines flight or 2,000 Alaska Airlines Mileage Plan bonus miles."

As for technical details, I interpret that to mean if "my" bag isn't there within 20 minutes of the seatbelt sign going off (which it can't do at the gate unless the aircraft is parked), I get the certificate. Doesn't matter whether it's on some belt in the inner-workings of the airport.

http://www.alaskaair.com/content/tra...guarantee.aspx