Every time a young blacksmith comes to my shop to visit or purchase an anvil I show them how I swing a hammer with almost no effort, and without hurting myself. I have wanted to do a serious video about this for years, but its a big project and perhaps no one is interested.
So here is my gift: a glimpse of this complex and fascinating subject. If you look closely at what I am doing you will see a lot going on that I don't explain. I am using a light hammer with a very long handle, but the technique works just a well with a 3 or 4 pound hammer. In a pinch I will swing a sledge hammer with one hand - same technique!
There are some popular hand hammering theories out there that differ greatly from what I advocate: super heavy hammers, and short handled clubs, rocking torsos and hands above the head. The "thumb on top of the hammer" perfect control idea is ancient. A lot of beautiful work is done in ways that I have left behind or perhaps just don't understand. I love you anyway.
If you get tired from hammering, or something hurts, please consider that there might be a better way. I remember going to my shop half exhausted one morning, but then I got into flow, and prepared and made 90 forge welds. 12 hours later I went home relaxed and refreshed.