Air pressure in the bottle pushes water (fuel) out of the small neck at high pressure. typically 80 to 120 PSI.This cause reaction which pushes the bottle in the opposite direction (See Newtons laws) the bottle flies until the fuel (water) is gone or the pressure is lost. Homework??
Water spray. Rocket go. Fast, fast, fast! Zoom, baby, zoom!
Exactly the same as solid fuel, liquid fuel rockets. Just a different approach. Google rocket theory.