It’s called the Folding Advanced Stopped Tilt Rotor (FASTR), and the basic idea is that you have some sort of variable cycle engine which powers tilt rotors on takeoff, and after you get to speed, the rotors are stopped, folded back, and the engines convert to turbojets.

I’m inclined to believe that this won’t amount to anything, as the weight penalties of a stop start folding rotor and variable cycle engine would likely make any advantages irrelevant, but it’s DARPA’s job to try out weird stuff like this, and see what happens.