If you see this videoat 0:42 you will see both of the pilots pushed a button on the front panel. can someone tell what that is and what it does?
They're starting the chronometer. This chronometer is displayed on the navigation display.
It lets them keep track of the time limit they are allowed to operate at takeoff thrust. The limit is 5 minutes normally extended to 10 with an engine failure. After this, they must reduce power to the maximum continuous thrust setting (MCT).
There is also a time limit between the setting of thrust and when the engine reaches commanded power. If it is too long, it indicates a malfunction and takeoff should be aborted.