SchoessI think you are trying to make this more concrete than it is. Jump trick axis's vary a lot from skier to skier. That's what makes them cool. Everyone cork 3's different from one another. Many people have cork 7's that are wildly different from one another. It doesn't make it a different trick. You did an underflip, you just set it weird and it's almost at the limit of a cork5ish rotation axis. It's also really hard to claim it's something new or different without doing it on a real jump. The way you have to throw it on that jump is out of necessity to get it around, not because you invented a new trick.
Just to be clear, I do not claim that this is a new trick. As you observed, I'm an amateur skier having fun trying tricks in its backyard. I set my goals, try new tricks, improve some, and enjoy!
I understand that there is this very fun level of freedom in freeskiing that makes almost every trick different. In comparison, in gymnastics, every "available trick" is precisely defined in an offical Code of Points and you're not allowed to diverge.
However, I feel that naming things (to a certain extend aka without overdoing it) have numerous advantages and that's the reason why I asked the question here. When you're new and learning tricks (as I do), having names helps you find the informations you need to try and improve the trick you have in mind (by watching tutorials, etc.). I think it's also cool to have names for things when you exchange with other riders and challenge yourselves etc. I'm not saying that we absolutly need to differenciate all the ways you can spin in any way possible, but there are some keywords (e.g. Flat vs. Cork spin) that can be helpful in all kind of situation.
Overall, I feel totally ok with the fact that sideflip/backflip motion with an extra 180 is called an "underflip". The question raised in my mind because it felt so different to me that I never thought about the fact that they could be the same trick. But in the end, I understand that for more experienced riders, it's just not important and that it might sounds like I'm trying to "make this more concrete than it is".