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Currently, the system is opt-in and limited to travelers who are frequent travelers or already members of a pre-screening travel program, such as NEXUS, Global Entry, or SENTRI. Once enrolled and screened, you gain access to a faster line where your shoes can remain on, your laptop can remain in the case, and you can keep your belt on while moving through security.
If you're interested in signing up, there's more information available on the TSA website or you if you are flying United, you can always try to sign up with the link that they sent me. Signing up does not guarentee acceptance into the program.