I have one, its a regular claw hammer, no it doesnt weaken the fabric, just pounds it down so its not so bulky.
I call mine the " persuader". Funny thing about it is, my sister sews for people, and she told me, when she hemmed jeans for people, that she used a hammer on the seam, to make it lay down, I'm like yeah ok sure. Next thing you know I was making pinwheels and the middle was a bit too bulky- a smack or 2 with the hammer, and its flat as a pancake!- PROBLEM SOLVED!!
So- have a whack-a-doodle day!
ya might as well have fun with it!
katz