The traditional definition of a pressure point is a point that, when pressure is applied, produces crippling pain. This is learnt in a chinese martial art called Dim Mak based on acupuncture pressure points,but this art is very restricted and needs and understanding of Chinese acupuncture points. When using these pressure points one must be particularly careful as it is easy to kill someone accidentally, such as a friend or even an enemy.
Striking the flat of the forehead forces the head back with little resistance and will actually rock the brain within the skull, causing a concussion, or worse. Beginners should use the heel of their palm, rather than a fist. The same holds true to the back of the skull, just below the horizontal ridge. (The front has one too, above the flat.) The ridges are strong enough to be used as weapons in their own right, so avoid them.