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Why it works
Pinning the f3-knight is a standard Benoni idea and 7...Bg4 develops with a threat, so it looks like the sharp choice. It ignores a hole on the a4-e8 diagonal that Black opened himself with 3...e6 and 6...g6. 8.Qa4+! forces 8...Qd7 — the only block that does not drop material outright — and 9.Bb5 piles onto the pinned queen. After 9...Bxf3 10.Bxd7+ the queen is simply gone. Fewer than one player in ten walks in (9% at 1600), which is why the tag stays modest, but everyone who does is losing the game in the opening.
Refutation
7...Bg7 is the main line for a reason — 86% of players choose it and score 53%. Finish the fianchetto, castle, and only then look for the pin: in the Benoni the king on e8 is the piece that makes ...Bg4 unaffordable.