The Transportation Security Administration’s new rules for screening passengers with its controversial full-body scanners — which were quietly changed just before the busy holiday travel season — represent a significant policy reversal that could affect your next flight. Getting checked by the TSA’s advanced-imaging technology used to be entirely optional, allowing those who refused a scan to be subjected to a pat-down. In fact, many observers thought the agency installed the 740 body scanners in 160 airports with an understanding that no one would be forced to use them, ever. But on a Friday in late December, the TSA revised its rules, saying an “opt out” is no longer an option for certain passengers. (The full document can be found on the Department of Homeland Security’s website.) The decision drew mixed reaction from experts and raised concerns from passengers. Christopher Elliott has posted a guide for perplexed travelers at Huffington Post. To see what you can expect on your next trip to the airport, click here.