This is an extreme listening exercise. The two players are expected to speak and the same time and literally talk over each other. While talking over is a habit to be avoided in improv comedy it is encouraged in this exercise. The simplest form Double Opening the two players will simultaneously yell out nouns at each other. It is very challenging to hear what the other player has said.
In a Line Gag the players are called upon to make short quips relating to the gag type chosen by the leader. We will use the storied example of 99 strings. In 99 Strings there is a very structured format where players make gags about nouns getting kicked out of a drinking establishment.
Players are assigned a broad category to reference. The categories must be broad. For example, cars, toys, cutlery, food, etc. Then they make endlress references to the category.