The specific energy drink flavor they're going with is Rockstar Punched, which as the name suggests is a fruit punch flavor.
The energy drink seems a good fit for the late night crowd at Taco Bell and the chain has long been associated with Mountain Dew (and Mountain Dew Baja Blast), which tends to have more caffeine than most sodas (they even did a Mountain Dew-orange juice blend for a while). Considering that they've already done Snapple and Starburst co-branded Freezes, it's more of a surprise that this didn't come about sooner.
