i think you're asking about the impact foam used in the helmets... yes, they are designed to absorb and deflect the energy so that your head/brain takes less brunt.

and you are right about the concussion. its the brain that hits the inside of the skull... but if the energy absorbing foam can take away enough crash energy, it could prevent a concusion if the force was low enough...

the most important thing is the correct fit. the padding is there for the fit, not necessarily impact absorption. if there is space/air gaps the your head will move inside the helmet and add more to the damage...

if you look at the pro racers, when they take off their helmets, you see a faint impression on their cheeks where the padding has been squeezing on them. thats the proper fit.

and just like the good doc stated, M rated helmets are good for only 1 impact. SA rated helmets are for mulitple impacts (since you can be thrown around in a car and hit many things). In either case, after one accident, its time to get a replacement helmet.