I learned to do this style years ago, I was taught to use a slightly spread flat 6. The hand movement is a little tricky...it's sort of a whip shade; but the machine is almost 90 degrees like a lining position, and the whip movement is like when you are whipping off a thick line, to get the progression from darker to lighter you make your whips closer together(for example pretend this symbol @ is the solid fill this symbol . the whip pepper and this symbol | the needle. A whip from the solid would look like: @|....... to get that darker you start the next whip around the starting point of the first like: @|..|..... as you shade further out you increase that space for lighter like: @|..|....|......| you can make the shading pattern consistent or random for different effects. I hope that makes some sense to those wondering. Hanging out the needle a lot so you don't really pack too in much ink is almost mandatory.

That's how I learned that type of shading, Lenert may have a slightly different approach to it. Once you figure out the mechanics you can tweak little things to fit what you like; such as angle, needle hangout etc. But I gave the exact start point for learning it the same as I was taught. But the needle is a slightly spread flat, if you haven't ever used a flat take care not to linger because it will cut a trench super quick, the spread helps prevent that somewhat.