Rivian has an EV technology stack that Volkswagen thinks has real value.Rivian Automotive (RIVN -8.15%) stock shot higher after it announced a surprising new deal with global auto giant Volkswagen earlier this week. As investors digest more details of the multifaceted arrangement, some are taking quick gains from the stock's jump.Today, Rivian shares were lower by 7.7% as of 2:45 p.m. ET. At that time, the stock was still up nearly 30% for the week. So what are investors to make of the volatility in Rivian ...