Puraista [puraisevat] is a verb meaning a quick, one time bite. Biting points to repetitive and/or continuous/ long lasting action, which would be translated as purra [purevat].
We don't have such distinctive (one time/ continuous) words for every action and that's why most Finnish verbs can be translated to English either way.
I'm really grateful to people like you who leave these helpful comments. I can't wait for duo to make lesson tips about this, it's one of the more difficult things to grasp for foreign speakers imo and consistently translate correctly (especially for those of us who aren't native English speakers either and may not have an intuitive understanding of verb tenses)
I'm afraid the way Duolingo has designed it, some of the learning occurs right here, in the sentence discussions... In the beginning, they didn't even have any Tips&Notes!
It's always worth checking these discussions, doing a Google search, asking a question in the general Finnish discussions, and/or consult reputable websites like uusikielemme.fi and wiktionary.
Yep, you're right about it being "too light". To nip would be closer to (näykkäistä)[https://en.wiktionary.org/wiki/n%C3%A4ykk%C3%A4ist%C3%A4].