This mechanism was developed to provide automatic staging of multi-stage water rockets. This mechanism will release a sustainer with a standard 9mm Gardena nozzle when the booster stops producing thrust.
The following tutorial describes how to build the staging mechanism. It should be used more as a guide rather than an exact step-by-step procedure as it is likely you will not have access to the identical parts used here. Feel free to substitute components as you see fit.
NOTE: Read the tutorial carefully including the notes at the end as there are important bits of information that you will need.