Pressurisation Bumps
Notwithstanding some of the positively surreal posts on this thread, here's the explanation of pressurisation bumps...
Many/most simpler old-school pressurisation systems connect the squat switch into the pressurisation system - often as a direct connection to the outflow valves, independently of the controller. That very simply satisfies the requirement to dump cabin pressure when you land. But the inverse is also true, that during the takeoff roll, wheels on the ground, the outflow valves are held open. As soon as the aircraft lifts off the squat switch opens, and transfers control of the outflow valves to the pressurisation controller. This handover of control can mean that the outflow valves are quite quickly driven closed as the controller hunts for the right setting, causing a noticeable "bump" in the ears of the passengers, especially if the controller is not quite set up correctly.
To answer the 747 comment - the size of the bump depends on cabin size - the smaller the cabin, the bigger the bump.
There are two pretty basic solutions. One is to arrange that there is no significant inflow when the outflow valves take over - that's the "no packs" solution, and it works - but the bleed air must be introduced to the cabin carefully later. The second, which is more popular in modern designs, is to give control of the outflow valves to the pressurisation controller when you are still on the ground. The controller can drive the outflow valves towards the closed position to modestly pre-pressurise the cabin. It can do this without bumping because it KNOWS that you are on the ground and can therefore self-calibrate the delta pressure between ambient and target cabin pressure.
But it has nothing to do with attitude, ram air, or airflow.