Yak shaving is a term for the seemingly unrelated things you end up doing while trying to accomplish some other task. I found myself with a delightful example while writing today.
A character has been slippery. Very well, who is this person? I'll digress and figure her out, tell her story to myself, so she can walk into my novel a breathing creature.
This implies significant yak shaving even writing on Earth unless you're writing about the town you live in, but as soon as you create a fictional world, you must digress from the character to construct the society that produced her, her family and upbringing. They, in turn, require a setting to live in. This is how worlds grow vastly beyond the small number of settings a novel actually traverses.
On a planet's surface, the construction of areas, the effects of geography and history, are fairly familiar to most reasonably read and traveled people, but this book isn't on a planet. Suddenly I find myself reading about orbital velocities and composition of trans-Neptunian objects.
The trick is minimizing it. The physicist in me wants to calculate transfer orbits and slingshots and velocity matching and the transfer orbits to send material insystem and the probable system of volatile materials futures used to hedge materials being sent in this way. Thas way lies madness and no character. I'm writing a novel, not a mathematics text (though there is that idea I have for a series of love letters involving Bessel functions). To quote Henry James, "Up to what point is such and such a development indispensable to the interest [of the subject]?"[preface to Roderick Hudson] and he was talking about the book itself, not the yak shaving of character predevelopment.
I chose to shave just enough yak fur off the astrophysics to give the character a setting, and I'll adjust my narrative to eschew any details. After all, what reader wants to hear about the numerical calculation of chaotic transfer orbits from a position leaving a trans-Neptunian dwarf planet's transfer orbit to to a cometary core in the inner Oort cloud?
Well, given my friends, probably quite a few, but they will have to be disappointed.
It was all worth it, though, when I traced the chain back out to the character herself, and as I told myself her story there came a moment when she sat up and drew breath in my mind, no longer a role in a narrative but a conception of a fellow human being.