Thanks for the feedback, it means a lot. That level selector tbh is some kind of bug-to-feaure product, but turns out it helps people to be able to go through our full game, so I kept it.
As for slingshot, think of your hands as the two ends of a slingshot, and your body as the projectile. The connection between them is elastic, so when you pull back and release your body, it snaps forward. If you release your anchored hands at that exact moment, goo will fly through the air!
I'll update a step-by-step tutorial about the slingshot mechanic in the game page to make things clear.
Hope this helps!