The Knock Up Stream is a tremendous happening of nature in the world of One Piece that has large significance within the Skypiea Arc and, in general, the series of world-building. This powerful vertical water current pulls things, as big as ships and objects, from the Blue Sea over to the White Sea, where sky islands like Skypiea are located.
An element in the current of high pressure of water, the Knock Up Stream is what occurs periodically in some parts of the One Piece world. This is an irregular and infrequent act; thus, it makes it a dangerous and most pursued phenomenon for those trying to reach the Sky Islands. They say it occurs once every few months, if not years.
The Knock Up Stream functions as a general combination of natural forces. The stream starts out from the accumulation of pressure from beneath the ocean floor, occurring in areas of huge geological activity. When the pressure becomes critical, it erupts in one huge vertical column of water, which can go as high as 10,000 meters. This eruption is powerful enough to launch ships and even large chunks of the ocean floor into the sky.
The Knock Up Stream is seen as a grand and horrifying sight evidenced in both the anime and manga. It is shown as a really large, massive pillar of water that connects two huge bodies, hence tending to rise into the sky. This visualization entails a lot of swirling clouds and high winds connected with it. The scale of it and the feel of power that it gives rise to call for some danger with awe for both the characters and an audience.
The Knock Up Stream is an event and a force in the adventure of the Straw Hat Pirates towards Skypiea. In episodes 152 to 153 in the anime, this definitive one takes into account the experience the team has as they ride the stream aboard their ship, Going Merry, up to the White Sea on their journey. That moment became the very highlight of the Skypiea Arc, with very courageous crews showing the unpredictability of the Grand Line.
To withstand the Knock Up Stream, ships must come up to a certain speed and positioning. This becomes part of the skill and timing in what is already a deathly task. In the series, the Straw Hats modify their ship under the assumption of holding up against the enormous pressures and forces inside the Knock Up Stream.
The concept of the Knock Up Stream interested so many, to the extent that fans wanted to draw physics and possibility in the real world from it. The phenomenon is, in fact, largely fantastical but is inspired by real oceanic and geological events—underwater volcanic eruptions and extreme vertical currents.
The Knock Up Stream serves several narrative functions in the overall One Piece storyline in such a way that relationships between blue seas and sky islands are able to receive a full emphasis on the vertical aspect. This, of course, is an eco-system world-building, and Oda does it in a way such that it gives his series a lot of literal and also metaphorical depth.
The Knock Up Stream also creates potential future-story ramifications. Some in the fandom like to believe that it may even factor in trips to other great locations or in discovery processes aimed at the history of the realms. This speculation is warranted because in the series many issues were at the introductory stage but are again revisited and further elaborated upon.