The definition of yak shaving is that shaving the yak is a prerequisite to the task at hand. You can't continue without doing it.
Of course, just because this appears to be the case doesn't always mean it's true. Plenty of times I've been waiting on X to do Y and I've managed to make significant progress on Y just by pretending that X is already done, and working on Y until X is actually necessary.
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The definition of yak shaving is that shaving the yak is a prerequisite to the task at hand. You can't continue without doing it.
Of course, just because this appears to be the case doesn't always mean it's true. Plenty of times I've been waiting on X to do Y and I've managed to make significant progress on Y just by pretending that X is already done, and working on Y until X is actually necessary.