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Why it works
After 4...Qa5 Black is committed to activity, and 5...Bg4 looks like the follow-through — a pin, a threat, a developing move all at once. One check unravels it: 6.Bxf7+! Kxf7 7.Ng5+ opens the d1–g4 diagonal with check, and 8.Qxg4 collects the bishop with a pawn as interest. From there the measured game ran 8...Nf6 9.Qc8+ Ke7 10.Qe6+ Kd8 11.Nf7# — the king dead in the center on move eleven. At 1400 the pin tempts 22% of Elephant players; they score 40% for it.
Refutation
5...Nc6 keeps developing and holds the best practical score here, 45% — though the honest note is that the Elephant is a one-pawn loan the position never repays. If you must play it, keep the light-squared bishop home until the king is settled.