From Harley-Davidson to Bank of America, Warren Buffett's investments are known for lighting a fire under weak stocks. The latest in need of help: his own company.

Having underperformed the S&P 500 so far this year, Berkshire Hathaway stock jumped 8.1% Monday when the company said it would repurchase shares - but only up to a valuation of 10% above book value. The move closed most of the valuation gap instantly, taking the shares to 1.08 times.