Just one sentence from Bernanke and the US stock market jumps

This morning, Ben Bernanke, gave a speech to the Council on Foreign Relations, a Washington think tank. During his presentation, he said:

"I think there is a good chance the recession will end later this year and 2010 will be a period of growth."

Within one hour of trading the Dow had surged by 262.28 points to 6,809.33 . So what exactly has changed?