The Pitot/Static test unit is used to verify air data instrumentation and system interconnects. Proper operation of the tester is necessary for functional checks of altitude/airspeed related components. The unit is also helpful with troubleshooting of autopilot, pressurization, and flight control limitation systems.
When I use the tester for troubleshooting or functional checks, I like to have the unit in the cockpit while tests are run. This sometimes involves having numerous test hose segments to reach the static port, but direct observation of aircraft indications is preferred for comparison. I also like to verify that my test box inputs are not exceeding the component limits.
Digital equipment on newer aircraft is much more forgiving than older analog components that use mechanical bellows for measurement. I still practice techniques that would be used on analog components regardless of what is installed.
Some important airspeed basics:
Airspeed indication is derived from both pitot (pressure) and static inputs.
If there is no difference between pitot and static pressures, airspeed will remain at 0 knots.
I like to perform checks in two steps. Pitot first, followed by static. This simplifies the difficulty of trying to keep the airspeed in limits while the aircraft is at altitude. The crossbleed valve enables an aircraft system to be driven to all operational altitudes without concern about airspeed exceedance.