Nishio asks one sharp question: if this candidate were the answer, could the rest of this digit still be placed legally? Put the digit in mentally and propagate only that digit's placements across the grid. When the trial walls itself in — some row or box left with no legal spot — the assumption was false, and the candidate is eliminated for good.
Assuming 8 at r2c2 (red) forces the blue 8s — and leaves the bottom-left box with no home for 8. The assumption dies.
Digit 8 has six placements already; test the candidate at r2c2. Propagate only the 8s: the trial forces 8 into r4c5 (last spot in its box) and then r7c8 — after which the bottom-left box has no legal cell for an 8 at all. Contradiction. The tested candidate was false, so the 8 at r2c2 is erased and everything you imagined is rolled back.
No contradiction proves nothing — it does NOT confirm the candidate, it only ends that trial. And propagate strictly the tested digit; the moment you chase other digits' consequences you've drifted into full trial-and-error.