Comment by icegreentea

They compare the degree of correlation between their identical and non-identical twins. If in both groups, twins show on average the same level of procrastination/whatever trait, you can infer that the trait is not strongly genetically linked. If the identical twin group shows more correlation between siblings than the non-identical twin group, then you can infer that the trait is strongly genetically linked. You can read more under the first few paragraphs of their data analysis section.