The bubbles go up, instead of down, because they weigh less than water. When the bubbles get to the surface of the water, they break open. All that gas that has escaped from the bubbles pushes on the sides of the canister. Now when you blow up a balloon, the air makes the balloon stretch bigger and bigger. All the water and gas rush down and out, pushing the canister up and up, along with the rocket attached to it.
Real rockets work kind of the same way.
But instead of using tablets that fizz in water, they use rocket fuel. Delta rocket similar to the one that launched the Deep Space 1 spacecraft from Cape Canaveral, Florida, in October 1998. Some pushed the rocket off the ground. Then some helped it continue its climb into space. Others gave the Deep Space 1 spacecraft its final push away from Earth.