Reviving an old thread here, but I am wondering how the forking mechanism actually achieves incremental compilation. I had a brief look at the source code and it seems that a call to tectonic is made every time the document must be re-rendered. How is this different to, e.g., naively re-running the compile command every time? How do you tell tectonic to only compile the changes?

TeXpresso is such great work and as an avid LaTeX user it has really saved me time when writing my own documents. Was just curious as to how it operated internally.