Polystyrene is very weak in tension (can be broken by hand) so won't be deflecting much energy. The principle is to slow the deceleration of the head by the polystyrene compressing and thus reducing the g-forces to the skull (not so much the brain which tends to slosh in the skull and cause concussion). Some motorbike helmets use materials such as polycarbonate which are intended to provide protection by breaking - quite different to bicycle helmets.