Start by examining every check. (Again, it's drill.) There are three: Nxa7, or a move of either knight to d6. Nxa7 loses the knight to NxN without a good follow-up. Moving either knight to d6 results in BxN, but White then can recapture with another check: NxB+. With his ability to capture on d6 now exhausted, Black would have to move his king out of check to b8 or c7. Then what? You could look for yet another check using White’s bishop, with inconclusive results. Or you could notice that White’s remaining knight then would be on a dark square and that Black’s rooks both would be on dark squares, too. Nf7 forks them and wins the exchange. This time the point of the checks wasn't to move the Black king into position to be forked. It was to keep Black busy with threats he had to address while you prepared to fork his rooks.