Monkey see monkey do, I always smashed it with the flat edge of a wide knife, because cooking shows, prior to pressing or grating.
However today I was in some deep thought and I realized it might be hurting my wrist a bit to slam on the knife with my palm.
First of all, are we talking about a whole garlic bulb, or a single clove?
If it's hurting your wrist to smack on a single clove with a knife, then I think you are doing it wrong. It should just need the gentlest of taps. The thought once occurred to me to try a kitchen mallet, but as I suspected it was like using a sledgehammer to crack a nut. Complete overkill, and it was in danger of obliterating the garlic in the process.
Important things are to be using the flat of a big chef's knife, not a small utility knife, to be working on a solid chopping board, and to cut off the root end of the garlic clove before tapping it. Then the technique is to give it the tiniest of love taps, just enough to loosen the skin so it comes off easily. If you actually squish the garlic you have hit it too hard.
As far as breaking the bulb apart is concerned, I just peel off the outer papery layers of skin, and then split the cloves apart with my fingers. I have seen someone smash a whole garlic bulb to break it apart, but that's only useful if you want to use the whole garlic in one go.