3 ideas
Restricting yourself to the thousand most common words makes jargon impossible, so every technical term has to be replaced by a description of what the thing does or what it is made of. A 'nuclear reactor' becomes a 'heavy metal power building' and 'Saturn V' becomes 'Up Goer Five.' The constraint exposes whether the explainer actually understands the mechanism or has only been repeating its labels.
Technical names like 'mitochondria' or 'helicopter' pack a whole mechanism into one opaque token, so a reader can use the word without knowing what the thing does. Renaming things by their job ('tiny bags of water you're made of' for cells, 'sky boat with turning wings' for helicopters) makes the underlying purpose visible and shows how parts relate. Seeing objects by what they do, not what they are called, turns a list of terms into a working model.
The drawing showed that even a moon rocket can be explained part by part, and that the result is both funny and genuinely informative. Its success showed that the constraint could carry a whole book of complex systems.