When I participate in a trading contest, I do not take any additional actions other than registration. I just keep trading, as I do every day, except in cases where the terms of the competition require trading certain pairs of coins.
Therefore, apart from registering alone, you also take time to read the rules (which are important) even though we are sure that the rules are still the same because we might be wrong with our assumptions and that will be the reason for our disqualification. It is seems you are an experienced trader already. That's great but for those who aren't yet and are excited to join their first competition, pretty sure they will also do a practice.
As you said, there are also trading contests that focus only on a few pair of coins. One of these coins must be new and it was like they are only promoting it. The rewards in the contest can also be sponsored by them (dev/team of the coin).