The goal is to encourage the yellow snail in the top lane, which is done by pressing [Z] every time the exclamation mark above the snail isn't visible, to win the race. Encouraging the snail as she crawls across the track makes her go faster but also stops her in her tracks. Too much encouragement, however, causes the snail to flip over and light on fire; encouraging her too little makes her believe her best was not enough.
If the yellow snail manages to win the race, the protagonist only receives 9G back as Napstablook needs to make a profit. If the snail loses by a slim margin, Napstablook observes that she thinks she won and rewards 30G so as to not hurt her feelings.
To win 30G consistently, encourage the snail 6-7 times at the start of the race, whenever the exclamation mark isn't present. When she gets near the finish line, encourage her constantly to effectively stall it in place.
- The minigame is likely a reference to the SpongeBob SquarePants episode, The Great Snail Race. In the episode, SpongeBob enters his snail Gary into a race, but because of being overworked before the contest, and being pushed too far during the event by SpongeBob's abusive "encouragement," Gary veers far off course and literally bursts into flames.
- The name of the game, "Thundersnail" may be a parody of the name "Undertale," seeing as they both rhyme.
- If the protagonist does not have 10G, Napstablook still allows them to play, taking whatever money they have.
- On a Genocide Route, the protagonist is unable to play this game, as both Napstablook and the snails are absent.