Short clip from the last hammer and hardie workshop. This shows a common error, not flexing at the wrist means the forearm remains too stiff. The 3rd strike shown here is exactly what you want, compare this to the other strikes and notice how much less the wrist moves. This is understandable, if you think of how you usually use a hammer, you do keep the wrist locked. It's easy to overcome this and generally it only takes a couple of weeks to loosen up the wrist enough.