FORTUNE — He’s done it again.
In August, corporate raider Carl Icahn announced on Twitter that he believed Apple (AAPL) at $470 was “extremely undervalued.” He said that he had taken a “large position” in the company, had talked to Tim Cook about accelerating the company’s record $60-billion stock buyback program, and would be meeting with Cook to discuss it further.
On the strength of two tweets from Icahn, the stock soared and the company gained $17 billion in market value in just over an hour.
Icahn issued another tweet Tuesday morning at 10:24 a.m. Eastern:
The effect on Apple’s share price was nearly instantaneous.