To be sure that your team has understood the activity they are about to carry out, don’t ask inefficient questions, such as “Have you understood?”. Nobody wishes to stand out by answering “NO”. “Have you got any questions” already sounds better. “What questions are there” sounds even better.
However, the best way to know if your team has understood everything they have to do is by asking questions that oblige them to go through the rules or steps once again. For example, you can ask “how many minutes do we have for doing…” or “What happens if you drop any of the balloons?” etc.
When someone has to win because that is the way an activity must end, make sure that all participants comprehend and comply with the criteria used to establish who won and who lost. If that doesn’t seem to be clear, ask a review-question, e.g. “How can one win this activity?”. Also, be sure to explain to participants what happens if activities between two teams, for example, end up tie.
You will have the final draw on designating the winner and be sure to let you team know that. It is better that they discuss your decision with you rather than start to argue with each other and lose sight of the true purpose of the activity.