Overview
Frivolous theory refers to theory being read against arguments that aren't very abusive. Frivolous theory is often considered unnecessary, but it allows the debater reading it to have an additional route to the ballot. Debaters who are proficient at theory might try to find any violation that is even marginally unfair just so that they can read theory.
When theory is evaluated under an offense-defense paradigm (i.e. competing interpretations), frivolous theory can often be difficult to respond to, since it will usually be true that one debater is being marginally more unfair. The debater who is responding to the theory would need to find some way to justify why their practice is actually good, which could often be difficult, especially when some frivolous theory shells aren't even arguing that one debater was being actively abusive – they instead argue that the debater should have done to be more fair.
For example, some frivolous interps might be, "The affirmative debater must specify what role of the ballot they are using to evaluate the round," "The negative must specify the status of the counterplan in a delineated text in the 1NC," or "Debaters must specify which branch of utilitarianism they are using for their framework." Even though these interpretations are read against practices that aren't very abusive, you will notice that it is hard to come up with proactive reasons why violating the interpretation would be good, which would be necessary under competing interpretations.
Responding to Frivolous Theory
Example
Interpretation: The affirmative must specify a framework to evaluate the round through
Violation: They don't
The standard is shiftiness--1ars can reclarify the framework in the 1AR to delink out of all 1nc offense to moot all of our offense, making it impossible for us to win. Cx doesn't check--non verifiable since judges don't flow it
Obviously, this shell is frivolous. The neg could have asked the aff in cx for a framework, read their own framework, etc, but it's also difficult to respond to because there is no reason that it is bad to specify a framework.