诚心请教投坛高手

今天Yahoo Finance发了这篇文章：
finance.yahoo.com/news/4-lessons-from-warren-buffett-s-biggest-quarter-ever-123213258.html

其中一段话说：
“2. Buffett talks a big game on taxes but Berkshire plays by the same rules as everyone else. Most of the earnings increase was in the form of investment gains, largely from cashing out when Amazon (AMZN) CEO Jeff Bezos bought the Washington Post. By swapping out shares instead of cash Buffett was able to book a 100-fold gain in Graham stock purchased in the 70s without paying any taxes.”

俺不明白为啥Buffett “book a 100-fold gain in Graham stock purchased in the 70s without paying any taxes.” 哪位老师解释下，非常感谢。