The FLT/GRD switch is moved to GRD with the "after landing items" - as you leave the runway.
The outflow valve will drive to "FULL OPEN" very fast, and this "dumps out" the small positive pressure in the cabin, resulting in a momentarily high cabin rate of climb (remember, more pressure -> cabin "descends", less pressure -> cabin "climbs").
Try to open the outflow valve when flying at 30.000 feet and see what happens with the cabin rate .
PS: Are you aware of the folder "documentation" contained in the aircraft folder? There is a .pdf in there called: 3-Pilot Quick Reference Handbook. It contains the normal procedures for operation, and may answer a lot of your questions (as it also contains some annotation for WHY things are done).
Here is the excerpt for Chapter 6:
CHAPTER 6
AFTER LANDING AND PARKING
Taxiing the aircraft to the parking position or gate after landing can be very demanding, too.
It takes good crew coordination to keep situational awareness, in addition to performing
some steps to prepare the aircraft for arrival at the gate.
AFTER-LANDING ITEMS:
Reversers stow
Speedbrake down
Landing lights as required
• Usually turn off the landing lights and turn on taxi-lights and runway-turnoff lights
Flaps up
Pitot-Static-Heat off
• This removes heating from the pitots during ground operation, they would get too
hot without the cooling airflow.
FLT/GRD Switch GRD
• This will open the outflow valve, depressurizing the airplane so you can open the
cabin doors later on.
Strobe Lights OFF
• They could blind other aircraft/personnel
Engine start switches OFF
APU As required
• Only use the APU if you need it for electrical power or aircondition after parking. Its
loud and uses fuel!
Single-engine taxi consider