"SDOO" stands for self-directed (positional) opt out. This refers to the act of positioning oneself nearest the metal detector at an airport security checkpoint so that one can avoid screening by an AIT machine (either X-ray body scanner or MMW body scanner). It doesn't always work. There are THREE possible scenarios which can ruin a good attempt at a SDOO: 1) TSA "smurfs" block the WTMD with some type of object or themselves, 2) a TSA "moat dragon" demands you go through the AIT, or 3) both scenarios #1 and #2 occur simultaneously. The term "SDOO" is commonly encountered on frequent flyer forums, such as Flyertalk, where travelers discuss options for avoiding unnecessary radiation exposure or aggressive pat-downs at security checkpoints.
Hopefully I can do an SDOO today at LAX without the TSA smurfs stopping me! I sure don't want to get cancer from the radiation that the backscatter emits!