Often when searching you may want to limit your results to tasks that are "but firsts" or "and thens" of another task. You can do this by opening the advanced section of the search panel and entering the appropriate task ID in the "But first" or "And then" field.
Some of the default task lists, like the main To Do page, don't show tasks which have "but firsts" (that is, tasks which need to be completed first). You can do the same thing by entering the special value "nothing" into the "But first" field of the search panel. This special value also works for the "And then" field. Similarly, the special value "something" will find tasks which have something to do before or after them as appropriate.