At least as of July 2013, the appreciation in the stock price was due to the increase in the value of Alibaba and Yahoo Japan.
"But Yahoo’s turnaround remains very much a work in progress. Although Yahoo’s stock price has soared by a whopping 73% since Mayer become CEO, that gain is almost entirely attributable to investments the company has in two Asian companies, the Chinese e-commerce giant Alibaba and Yahoo Japan. Yahoo’s core advertising business remains sluggish amid intense competition from rival Internet giants like Google and Facebook."
http://business.time.com/2013/07/16/yahoo-ceo-marissa-mayers...