Based on their star-formation rates, galaxies can be classified as either star-forming or quiescent. The process by which star-formation in a galaxy is shut off is known as quenching. There are several mechanisms that can potentially quench a galaxy. Observations have shown that in some cases the quenching process is particularly fast, producing a special class of galaxies known as post-starburst. In this project, we use data from the IllustrisTNG suite of cosmological galaxy formation simulations to study massive post-starburst galaxies at z=2 and to explore the processes of galaxy quenching and transformation. The simulated post-starburst galaxies show some unique characteristics: we measure large sizes, in tension with observational claims; their age gradients are positive; and they frequently have central starbursts. These are distinct markers of unusual quenching activity. We also track the origin and fate of post-starburst galaxies to higher and lower redshifts, comparing the evolution of their age profiles to that of regular quiescent galaxies. Our results may have important implications for studies of massive galaxies, and may hold the key to understanding some of the fundamental mechanisms of galaxy quenching.