A helmet reduces the peak energy of a sharp impact. This requires a layer of stiff foam to cushion the blow. Most bicycle helmets do this with crushable expanded polystyrene (EPS), the white picnic cooler foam. EPS works well, but when crushed it does not recover. A similar foam called expanded polypropylene (EPP) does recover, but is much less common. Another foam called EPU (expanded polyurethane) has a uniform cell structure and crushes without rebound, but is heavier than EPS and its manufacturing process is not environmentally friendly. Other foams and deformable plastic systems appearing that may offer promise. The spongy foam pads inside a helmet are for comfort and fit, not for impact protection
The helmet must stay on your head even when you hit more than once–usually a car first, and then the road, or perhaps several trees on a mountainside. So it needs a strong strap and buckle. The helmet should sit level on your head and cover as much as possible. Above all, with the strap fastened you should not be able to get the helmet off your head by any combination of pulling or twisting. If it comes off or slips enough to leave large areas of your head unprotected, adjust the straps again or try another helmet. Keep the strap comfortably snug when riding. The straps hold your helmet on, not the rear stabilizer.