Investors watching the vertigo-inducing rise—and Thursday’s fall—of technology stocks are buzzing about a single trade, a giant but shadowy bet on Silicon Valley big enough to pull the market up with it.

The investor behind that trade, according to people familiar with the matter, is Japan’s SoftBank, which bought options tied to billions of dollars worth of individual tech stocks. Investors and analysts, aware of the activity but in the dark as to who is behind it, say it has turbocharged the tech sector, whose sheer size drives broader market moves.