A quick video explaining the "crossbleed" function of a Pitot/Static Box. Airspeed indication uses both ram air pressure and static pressure for indication. Increasing altitude while leaving pitot at ambient will cause the airspeed to rise. Opening the crossbleed (which ports static air to the pitot side, keeps the relative difference between the two systems at 0 and the airspeed will not react to increasing static vacuum while going to higher altitudes.
