This is the 11943rd most frequent Russian word.
затаив
"Затаив" means "concealing" or "holding back" in English.
Here, 'затаив' is used to describe an action (holding his breath) concurrent with the main action (awaited). It functions as a gerundial participle of 'затаить'.
Затаив дыхание, он ждал ответа.
Holding his breath, he awaited the answer.
In this sentence, 'затаив' expresses the action (suppressing her interest) while she was observing.
Она наблюдала за происходящим, затаив интерес.
She observed the events, suppressing her interest.
'Затаив' in this context conveys the manner in which the individual felt as he left the room.