Why is Nb5 a strong move here? Why is there a guaranteed material gain? Taken from:

Chess Academy: A subtle blunder

[Title "White to move"]
[FEN "r1bqr1k1/pp1nbpp1/4pn1p/2pp4/2PP1B2/P1NBPN2/1P3PPP/R2Q1RK1 w - - 0 1"]


White can play Nb5 followed by Nc7 but wouldn't Black play Rf8 beforehand?
This is a familiar tactic I often get to use myself in another opening line. If black plays 1... Rf8, then comes 2. Bc7! Qe8 3. Nd6 Bxd6 4. Bxd6 and black loses the exchange anyway.

[Title "White to move"]
[FEN "r1bqr1k1/pp1nbpp1/4pn1p/2pp4/2PP1B2/P1NBPN2/1P3PPP/R2Q1RK1 w - - 0 1"]

1. Nb5 Rf8 2. Bc7! Qe8 3. Nd6 Bxd6 4. Bxd6


Chess is about patterns. Since I was familiar with the pattern, I spotted this tactic immediately.