Why might tucking help a trans woman avoid the groin anomaly issue? Well, the machine is looking for something down there . . .
When tucked, a trans woman's physiology —in this case the contours of her groin— will better match the pattern for "female". When we say pattern, we mean it will match what the programmer at the TSA encoded into the scanner software, where clearly there was a choice made that defined "female" as a person without a penis. This binary software choice effectively erases trans, intersex, and gender non-conforming people, but that's a topic for another post.
For the real nerds, here's more on the TSA's "Advanced Imaging Technology"
"(AIT) is TSA’s term for devices it uses to scan the contours of the human body to look for things under a person’s clothing that might be dangerous items. All TSA scanners are now equipped with software called Automatic Target Recognition (ATR). ATR analyzes the scan image of the body and displays an outline of a generic person with the location of anything the software identifies as an “anomaly” or "alarm" that TSA agents need to look at more closely. With ATR, according to the TSA, images of a traveler’s actual body are never viewed by humans.
TSA will not disclose details of how ATR detects anomalies, however in some cases, ATR software can register body contours not typical for a person’s gender as anomalies. Foreign objects such as prosthetics, binding garments, or even paper or change left in a pocket will commonly register as anomalies requiring further screening. Often this consists of a limited pat-down of the area(s) where an anomaly was detected, however it can potentially involve a complete pat-down."
You may opt out of AIT scans at any time, but if you do opt out of AIT screening, you will be required to undergo a thorough pat-down.