schnellen is a verb that means something like to leap or to move with fast acceleration. It's sometimes used in predator-prey context, sometimes for springs (the bouncy-tings, not the season) but I guess the most common use case is still stock price reports.
„Als er den Abzug betätigt, schnellt der Schlagbolzen nach vorne.“ – „As he pulls the trigger, the firing pin springs forward.“
Be aware that I gave examples for the verb schnellen. This word is uncommon for a boy. But the Duolingo sentence uses the adjective schnell. And this word is very common, equally for inanimate objects as well as animals or people.
Duolingo's vocabulary tooltip is rather primitive when it comes to word lookup and ignores context (I don't blame them, as it's a complicated matter), so it suggested the spring translation for the word. But that is a possible translation for the verb, not the adjective.