It is an example of a [reduced] absolute phrase.
The following from grammar.ccc.com:
Usually (but not always, as we shall see), an
absolute phrase (also called a nominative absolute) is a group of
words consisting of a noun or pronoun and a participle as well as any
related modifiers. Absolute phrases do not directly connect to or
modify any specific word in the rest of the sentence; instead, they
modify the entire sentence, adding information. They are always
treated as parenthetical elements and are set off from the rest of
the sentence with a comma or a pair of commas (sometimes by a dash or
pair of dashes). Notice that absolute phrases contain a subject
(which is often modified by a participle), but not a true finite
Their reputation as winners secured by victory, the New York
Liberty charged into the semifinals.
The season nearly finished, Rebecca Lobo and Sophie Witherspoon
emerged as true leaders.
The two superstars signed autographs into the night, their faces beaming
When the participle of an absolute phrase is a form of to be, such as
being or having been, the participle is often left out but understood.
The season [being] over, they were mobbed by fans in Times Square.
[Having been] Stars all their adult lives, they seemed used to the attention.