I can't blame you for not remembering Crush for PSP, despite the fact that it was pretty great. It's been four years since the perspective-altering puzzler arrived on PSP, and while it would have been reasonable to expect Sega had long left Crush behind, that's not the case.
Today, we have CRUSH3D.
It's unclear how much of CRUSH3D is simply an extension of the mostly-forgotten Crush for PSP or a wholly original creation for 3DS. The press release says CRUSH3D includes "new locations, characters and an intuitive hint system," in addition to a gift-giving feature powered by StreetPass. At last, a new reason to aimlessly walk around with your 3DS.
Understanding Crush is easier via playing than description, but fundamentally, you're shifting your perspective around while also altering dimensions. With the click of a button, the 3D space becomes 2D, allowing you to traverse distances that would be utterly impossible in the 3D space and vice versa.