This vex forum thread should help. There are lots of YouTube videos which talk a lot about there catapult. ( A good search would be, “vex VRC catapult explanation”) I have some links below which are some catapult explanations, but remember that there are a lot more on YouTube and discord.
Soon up catapult with pto exsplanation
Another form thread about catas
Pits and parts robot with a cata
Remember that most of the results will in fact be spin up. If you are looking for strictly over under then be sure to search that