I have had some questions recently on how to load the rocket, and how the collar works.
As a description, the zip tie heads, grip the collar around the neck of the bottle.
Air pressure tries to force the bottle/rocket up and off the launch tube. But so long as the launch collar is up, the bottle is unable to shed the grip heads.
Once the launch collar is pulled down, it allows the grip heads to spread apart which then allows the bottle to fly upward under pressure.
One might also ask why doesn't slide down before being pressurized. This is because while the launch tube looks straight with the same diameter it is not. The tube is flared by heat in the center. This creates a place of diameter wider than the bottle opening. This keeps the bottle from sliding down, and also forms an air and water seal for pressurization.
