Investors in master limited partnerships have just gotten the jolt of a lifetime. The Alerian MLP Index, a leading measure of the performance of these income-paying energy companies, fell 5.7% on Sept. 28 and another 5.8% the next day. At that point, the index was down 22% for the month and 36% for 2015. Then, over the next week, it shot up 20%. That bone-shaking volatility is just the latest evidence that there’s a huge difference between understanding your investments and thinking you understand them. MLPs, most of which produce, transport, refine and store oil and natural gas, have been one of the hottest vehicles offering higher income to yield-famished investors. For the 10 years through 2012, according to Alerian, energy partnerships generated an average yield — income divided by price — of 7%. Their total return — income plus changes in market price — averaged 16.5% annually. Funds […]