(Please scroll down to the bottom of the post to try the activity for yourself.)
This post refers to the optional text-based activity, Trapdoor, as introduced in this blog-post / user-guide:
http://textivate.posthaven.com/new-optional-text-activity-trapdoor
The end result of this blog post also requires you to include a parallel text translation in the L1 (which may or may not be divided into "chunks" by adding vertical pipes as in the example shown). See this user-guide on parallel texts and how to add them to your resource:
http://textivate.posthaven.com/parallel-texts-slash-extra-texts-along-with-a-textivate-resource