The NPR News item and podcast In Indonesia, Joko Widodo Secures Another 5-Year Term As President quotes Indonesian president Joko Widodo, and continues:
"Let us reunite as family. Let us strengthen our unity," he said. He urged patience until the official results were in. But the unofficial tally, which has proven to be accurate in the past, came with head-snapping speed. The system known as quick counts doesn't rely on asking people how they voted but rather projects the winner from a sample of the actual ballots marked. And it's all over just two hours after the polls close. You might call it a model of efficiency if you weren't the challenger. Prabowo Subianto says his own tracking showed that he had won and insisted that some polls had opened late.
This seems to suggest that the unofficial tally is not completely unofficial, in the sense that it requires access to a sample of actual votes.
Sampling theory is a substantial topic unto itself, it's important to sample correctly, and to combine data and interpret it with caution and a good understanding of the errors.
Question: How does Indonesia's unofficial presidential election tally work, such that it might possibly produce reasonably informative results and yet finish with "head-snapping" speed?
update: As noted in comments below, Indonesia has some notable aspects that are germane here; 17,000 islands, hundreds of languages, 810,000 polling stations and 6,000,000 election workers could make fast yet accurate tallying quite the challenge.