With Downtown as a backdrop, Danilo Packer, 21, the 3-time Brazilian national yo-yo champion showed how to make the toy defy the laws of gravity. He was one of many champions who came to the Seaport Saturday to compete in events organized by YoYoNation.com, “The Laceration Combo,” “The Cow Wrap,” “The Reverse Suicide” and “Split the Atom” were just a few of the tricks up the yo-yoers sleeves.