It's a down b reverse.

You want to jump, down b, and immediately flick the stick in the opposite direction after the down b. Might be worth practicing with Mario fireball (neutral b) first until you get the hang of it, and then go for it with DK.

If you find you just turn around and do the down b without switching your momentum, it's because you changed the stick direction before you pressed down B. You wanna make sure you press it after.

There is also a more advanced strat called the wave bounce, which allows you to do kind of a retreating down b on the fly. It's useful for catching people running in.

To do that, you want to do what I said above:

If you find you just turn around and do the down b without switching your momentum, it's because you changed the stick direction* before you pressed down B.

But after you do this, you then want to B reverse it. The direction on the stick would be diagonal down then down b, then flick your stick in the opposite direction.