The large backside basin on the moon (south pole-Aitken, SPA) was first recognized in the early 70s, but has been more clearly defined only recently from Galileo and Clementine missions. It presents a serious challenge to our understanding of the impact process for several reasons. First, nominal scaling relations would predict that such a collision should have disrupted the entire moon. Second, gravity and topographic data indicate that in spite of its enormous size, it apparently did not result in uplift of the lunar mantle as in other major basin-forming impacts. Third, a well-defined series of massifs has survived even though most of the structure has been destroyed. And fourth, the farside lunar highlands seem to be thickened at one end of the long axis. It is proposed that such enigmas can be resolved for a low-velocity and low-angle impact by an object whose radius becomes a significant fraction (more than 10 percent) of the impactee radius. At such extremes, fate of the impactor following first contact with the surface largely controls the total energy ultimately partitioned to the cratering process. In contrast with other large lunar basin-forming impacts, much of the SPA impactor debris failed to strike the surface downrange. Nevertheless, it is speculated that much of this debris may have contributed to the narrow range in ages of later basin-forming impacts, the late-stage heavy bombardment, and the lunar sample record of impact melt.