Update: My nephew's help on a small job revealed a simple trowel move
Honestly, my brother needed a quick patch on his walkway and I brought my nephew along. Tbh, the kid was just dragging the trowel lightly like he was drawing. Ngl, I saw it left a really even finish without any lines. I started using that gentle pull on my bigger jobs now. It saves my arm and looks pro.
Sounds like your nephew just outsmarted years of grunt work. Ngl, it's always the kids who find the lazy way that turns out to be the real technique. So much for throwing out your back trying to muscle it perfect. Guess the pro move is just dragging it like a crayon. Better hope he doesn't start charging you for the lesson, lol.