Helmets are designed to absorb the energy of impact before it gets transmitted to the skull and ultimately to the brain. However, there are several mechanisms of brain damage that can occur even with a helmet. One is a contra-coup injury where the skull stops moving but the brain continues to accelerate until stopped by the inside of the skull. This can damage the brain itself or cause tearing of the blood vessels between the dura (the thick non-elastic covering of the brain) and the brain itself. This will cause bleeding into the space between the brain and the dura resulting in a ?sub-dural? hemorrhage.
A good web site with information about brain injury is
http://www.tbiguide.com/howbrainhurt.html
For historical sake, President Reagan had a sub-dural when he fell off of his horse on the ranch.