This concept is actually a lot more relevant when you get into later lessons, but basically what you want to avoid at all costs is interacting with your drawing as though it is just a flat, two dimensional thing. Every change you make to it must be done keeping in mind the notion that you're interacting with something three dimensional. With leaves, this is harder to explain for the same reason that it's harder to do wrong - leaves themselves are flat, so there's a lot more you can get away with.

Where it starts to become a serious problem is when you get into more solid, organic and geometric forms. Cutting across a form's silhouette as though it's just a shape on your page immediately reminds the viewer that they're looking at a flat drawing. Instead, whenever we manipulate such a form, we need to always run our cuts along that form's surface, constantly reinforcing the illusion that it's 3D rather than undermining it.

Especially earlier on, it can be difficult for students to really understand the difference between cutting back into the 3D, physical leaf that they're drawing, or cutting back into its 2D silhouette on the page, so for now I recommend that students try to primarily create things like ragged, spiky edges by adding bits onto the form itself, rather than cutting back into it.

Here's a more visual explanation of what it means to apply subtractive construction correctly and incorrectly in organic and geometric forms: https://i.imgur.com/qVQXfIY.png