For someone's sake!! Let's answer the question FIRST, and then ramble on with crap. (How much blue language can we use here, by the way?)

The whistle notes go up in harmonics. That means that when you play a D fingering and blow harder, the next note is the octave, the next the fifth above (A) and the next - very hard - a third D. It is easier to play that D by fingering G and blowing the note above the octave. (This gives the fifth above G, namely D.)