YouRang: Friday 17...?
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This took me some time. The combination isn't that complicated, but it's long enough to make my board vision go fuzzy. Pf2 definitely seems to be the hot spot. I (black) have threee attackers, and Pf2 has three defenders, so based on that it's adequately defended. Of course, often things aren't so simple. Just counting attackers and defenders fails if the defenders are more valuable than the attackers. It also fails if some tactic emerges after the exchanges. Typically you start exchanging with your least valuable piece, so: <17...Nxf2>, and white would recapture with it's least valuable piece: <18.Bxf2>
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The next capture (in value order) is the black rook and white queen: <18...Rxf2 19.Qxf2>, and the position we need to visualize is this:
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So far, black has spent a rook to get this pin. Is there a payoff? Taking white's queen is no good, and neither is attacking it with ...Rf8. The only thing that looks promising is bringing my bishop into it at e4, and for that to happen I have to move my knight (so my Q is guarded by the bishop) and compel white to capture on e4. Black has the perfect move for that: <19...Ng4!> forcing <20.Qxd4 Bxd4+>
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Now it's clear that indeed a nice windmill tactic has emerged, which should be enough to get my rook back with a couple pawns better. | 
Kibitzing
sac: 18...Rxf2
(-1.90 @ 35 depth, Komodo 10.1). 
