At their sole discretion, and without any public guidelines, TSA agents can now opt you back into the full-body scanner, even if you opt out.
Though the document in which this policy change was announced includes a "principle of transparency" that states "There
should be no system the existence of which is a secret," if there is any guideline limiting when a TSO can force you through a scanner, that guideline is a secret.
TSA is updating the AIT PIA to reflect a change to the operating protocol regarding the
ability of individuals to opt out of AIT screening in favor of physical screening. While
passengers may generally decline AIT screening in favor of physical screening, TSA may direct
mandatory AIT screening for some passengers as warranted by security considerations in order
to safeguard transportation security
TSA Advanced Imaging Technology [Jill Vaughan/DHS]
(via Interesting People)