What is displacement venting?…Ever try blowing air into a bottle?

Can’t do it!….not unless you put a hole in the bottom of the bottle.

Venting a room is much the same. It’s only successful when the room has a means of allowing a volume of air to escape (being displaced) so as to allow fresh or cool ambient air to enter.
You open a door and the door will open at the rate of the displacement of air.
For instance opening a door in a room that has wall or ceiling vent is much easier than doing so in a sealed room.

Displacement

If you want the cool air outside to enter the room you need to provide a means of displacement.
Same as a bank account, you can only take it out as fast as you put it in.
In this case the reverse dynamic applies but the principle remains the same.

A means of efficient displacement is having the air move up through the ceiling into the roof void using a purge vent.
Then having the air escape out of the roof via an efficient roof ventilator such as the Condor system.
There are no security concerns when the purge vent is operational even when the house is left unoccupied

Condor Kinetic