In the Roald Dahl book 'BFG', what is unusual about the bubbles in the drink frobscottle?

The BFG (short for "Big Friendly Giant") is a 1982 children's book written by British novelist Roald Dahl and illustrated by Quentin Blake. It is an expansion of a short story from Dahl's 1975 book Danny, the Champion of the World. The book is dedicated to Dahl's late daughter, Olivia, who died of measles encephalitis at the age of seven in 1962.

Two of the main characters are:

Sophie: The imaginative creative and kind-hearted protagonist of the story who becomes a brave international heroine. Portrayed by Amanda Root in the 1989 film, and Ruby Barnhill in the 2016 film.

The BFG: A friendly, benevolent, gentle, sweet 24-foot-tall giant who has superhuman hearing abilities and immense speed. His primary occupation is the collection and distribution of good dreams to children.

During the novel Sophie announces she is thirsty, the BFG treats her to a fizzy drink called frobscottle which causes noisy flatulence because of the bubbles sinking downwards. The BFG calls this "Whizpopping".