The trick to getting the 3 second slush, is quickly releasing the pressure in the bottle and re-securing the cap, flipping the bottle upside down, and back upright again. This is because the forming ice crystals will be moved around the length of the bottle, and trigger nucleation for the rest of the soda.
If you were to just take the cap off, ice will form, and it will slowly spread downward, but might take upwards of 2 minutes for the bottle to completely freeze.
Once you've iced your soda, try pouring it into a glass to see how slushy it really is. It has a consistency similar to that of a Slurpee.
It's actually the water that is forming the ice crystals, and you'll notice the ice will begin to float to the top, trapping some of the soda syrup, and making for a delicious carbonated ice slush.
If you were to remove the ice, the rest of the syrup would be more concentrated due to all the water that was taken out from it.
I found that if you opened the cap just enough to hear the bottle hissing, and held it there until it stopped, you could remove the cap completely and the soda would stay a liquid