When regular chin - ups and pull - ups become too easy, many add weight by simply holding dumbbells between their legs or using weight belts. (en.wikibooks.org)
Whether bodyweight or throwing dumbbells between your legs, the same applies here as it did dumbbells. (onnit.com)
For example, using vests for squats are much easier than holding dumbbells between your legs. (homefitnessguru.net)