"Have a look at the cen testing criteria for pads,then have a look at the one for helmets.Helmets are made from eps with a thin covering (which provides no structural purpose) and rely on the material deforming permanently to absorb crash impact.Pads use a bit of plastic with some thin cushioning."
The materials are obviously different. The result is the same and the methodology is equivalent. All of this stuff works by taking an impact, and reducing the amount of force that comes out the other side. Helmets, knee pads, all the same but with different details in execution of course.
Permanent deformation isn't relevant- some body armour is single use, such as Knox's honeycomb back protector, which is crush-and-replace. Others are not, such as Knox's foam back protector, which is multiple-use. The only difference there is that one passes the level 2 EN for back protectors and one passes the level 1. Suomy have demo'd a non-deforming, multiple use crash helmet which exceeded the CEN for helmets for that matter.
EN1621-1 lays out exactly what a piece of body armour has to achieve to be legally sold as armour- simply, it has to take an impact of a specific power, and transmit a reduced force into the test body. And since it's this force that injures you, reducing that reduces the risk of injury. If the force is reduced below the threshold of a specific injury, then the injury is prevented. It's really not brain surgery. Or, you know, knee surgery. That's also exactly how helmets work.