Have confidence, you are an expert and a peer. Sometimes a presenting team will talk too quickly or put up lots of equations without explaining them. Do not be intimidated by this! If you do not understand something it is because the presenting team did not do its job. It is their responsibility to be clear and explain things. It is not your responsibility to know what an obscure equation means. It is unlikely the jurors know what it means either. You look smart if you ask for clarification. For example, you may ask the presenter to explain an equation that you do not understand. You can always ask what are the underlying fundamental physical principles.
Stick to the most fundamental physics. You are usually more familiar with the particular problem than the judges, but the judges want to know if you, and the presenter, understand the fundamental underlying principles. Thus, it is your job to ask clarifying questions that both elucidate the problem for the judges and demonstrate your command of the material. If you think you understand it, clearly explain it back to the presenter and ask for agreement. Remember, most everything in physics can be understood using the concepts of force and energy, so always come back to the most fundamental concepts such as Newton’s laws, and the conservation laws of mass, energy, momentum, angular momentum, and charge.
Use the white board. Draw out your ideas in a clear manner so everyone can see. If you have a question, draw it, like you were a physics teacher in front of the class. Start with something very simple, called a toy model, and then move on to the actual experiment involved. If English is not your native language, you can communicate just as well using pictures.
Come across as curious, not combative. Alliteration is one of the most natural rhyming schemes in English, so “physics fight” just sounds fun. Notice the “f” sound in physics, fight, and fun. This is the only reason we call them “physics fights,” because in English it sounds better than “physics discussions,” or “physics questions and answers.” However, you will score higher as the opponent if you appear genuinely interested in understanding how nature really works. You, and the presenter, both worked on the same problem for about a year, and so you should be genuinely interested in how they solved it.
Do not discuss any of your own work. The whole focus of the discussion should be about the work of the other team. You are interested in what they did, what they concluded, and the evidence that they have that supports their conclusions. You will be severely marked down for discussing your own findings, rather than for asking good questions related to their work.