You can experience this if you hit a coin with a hammer a few times.

So the flash could cook a chicken by slapping it

I read once that the Mongolian warriors would place raw meat under their saddles and after riding all day would then consume it. Now I'm thinking that's not so far fetched.

And what would that do to my hand?

There are so many weird assumptions here. There is more than a hand moving when a slap is performed.

A skilled slapper could put more of their body weight behind the slap. I'd assume at least 40 kg or even more as the average slap.

Not chicken, but someone tried hitting steak with drum pedals: https://m.youtube.com/watch?v=QFTCqnYk5Sw

How can she slap?!?!

Oh, that's how

Average rotisserie chicken is 2 lb? Costco's is 3lb. That would require many more slaps.

What if you just want to warm up some sausage?