I mean in this sentence "A buzzer sounds sharply in the distance. without looking up, he lifts his middle finger up on one hand and flips 'the bird' , holds it a moment.drops it." I dont know what he is doing.
He is sticking his middle finger up. It is an obscene gesture basically implying that one should initiate sexual relations with oneself.
In this context, he is telling the buzzer to "f--- off," expressing complete contempt for what it represents.
(I'd paste in the graphic, but it wouldn't be terribly nice to my fellow Americans reading the page. That said, if you drive the Beltway here in DC, it won't be anything new. )
I'm not an American, but I'm pretty sure it means sticking one's middle finger up, which is rather rude.
This is somewhat off-topic, but my ADD has kicked in and I must comment.
As a bilingual American (German is my second language), I find it interesting to note that in German there is an insult gesture where one taps the side of one's head or one's forehead, with the index finger while looking at another person. The gesture means "Du hast einen Vogel" ("You have a bird" -- presumably in your head), and signifying that the recipient has some mental problem. This is most frequently encountered while on the road driving a car.
A photo of two drivers "showing the bird" to each other can be found on the German Wikipedia, HERE. That the photo is from 1939 indicates that this is a tradition of long duration. Believe it or not, in Germany it is illegal to "show the bird" to another driver. One wonders how often it is enforced.