Answer: "An abrupt snap"
The use of the word "abrupt" gives context as to how the action was done. The previous phrase, has Tim saying that he will have to be the "grown-up". This is because he knows that his friends are swimming and having fun, while he is going to have to clean up for them.
This phrase could be read as if Tim was sighing, making him upset. That is the reason why he then proceeds to open the bag violently, showing that he's angry.