Octopuses normally swim by pulsing their tentacles, like the bell of a jellyfish. This is quite energy efficient but it lacks acceleration, so when an octopus needs a sudden turn of speed it switches to jet propulsion. To do this, they suck water into the mantle cavity and then squirt it at high pressure out of their siphon tube. This is the same way that they circulate water over their gills, so the octopus is really taking a deep breath and then blowing out hard. The siphon is positioned on the side of the octopus’s body but it can be steered like the engines of a Harrier jet. Sometimes they use it as a boost when walking as well.