1
general::
Move swiftly and smartly to an action, as in The troops snapped to attention. This phrase is sometimes expanded to snap to it, as in You'd better snap to it if we're going to finish today. [Early 1900s]
American Heritage Idioms
2
general::
Phrase(s): snap to (attention)
Fig. to move quickly to military attention. • The troops snapped to attention when they saw the general appear. • Snap to when I tell you!
McGrawhill's American Idioms And Phrasal Verbs