When I was a kid, I put an egg in a container of honey and dropped it from a 2nd-story balcony onto a hard surface. The egg remained unbroken. The honey was u

When I was a kid, I put an egg in a container of honey and dropped it from a 2nd-story balcony onto a hard surface. The egg remained unbroken. The honey was undamaged. The kinetic energy represented by the difference in height was still added to the egg, but the honey slowed the transfer down, and spread it out to the whole object, in the same way that walking the egg down the stairs, instead of dropping it off the balcony, would have slowed it down. You don't always want the helmet to be damaged. I wouldn't want my helmet to be constantly denting, cracking, spalling, and ablating every time it touched something. You really want it to be damaged only when the kinetic energy is coming in too quickly to safely (slowly, widely) transfer it all to the protected head, and permanent deformation is the fastest way to bleed off that excess. In theory, the helmet could also heat up when it gets hit, but that's more expensive than just wrapping EPS around whatever you want to protect.