This position closely resembles the last one; but here each side has an extra piece, and the one that matters is White’s bishop on c7. Now if Black tries the same beginning move discussed in the prior position—Ng4—White has a better way than Rd1 to fend off the threat of Ra1#. He can instead play Bb6, preparing to meet Black's Ra1 with Bg1, which would effectively block the check. So Black needs another idea.
The natural alternative to explore is an immediate 1. …Ra1+, forcing White’s king to h2. Black then looks for his next check and finds 2. …Ng4+, forcing the king up another square to h3. Now watch this: Black plays 3. …Ne3—not a check, but renewing the threat of mate via Rh1 since the knight now seals off g4 and keeps the White king trapped on the h file. White somehow has to find an out for his king. Moving it to h4 won’t do; that still leaves no safe square after Black plays Rh1. But White has two other choices.
(a) The first is Kh2, readying the king to capture the rook if it moves to h1. But keep track of the Black knight’s position: it has moved twice and is on e3, a dark square. If White does move his king to h2, another dark square, Black can skip Rh1; instead he wins a rook with the knight fork Nf1+.
(b) White’s other option in reply to 3. …Ne3 is to step his g3 pawn forward to g4, giving the king a flight square to the side. Now Black plays 4.