With a continuously running clock perform one pull-up in the first minute, two pull-ups in the second minute, three pull-ups in the third minute, four pull-ups in the fourth minute etc. . . .
Continue until you are unable to complete the required repetitions in the allotted minute or until you reach 20 minutes.
If you are unable to complete the required repetitions in the allotted minute before 20 minutes has elapsed, rest for 1 minute and then resume by performing the number of repetitions from your last successful attempt and reducing the number of repetitions by one each minute thereafter until 20 minutes has been reached.