Trigger-Operation is a type of self-loading system in air guns where pulling the trigger chambers the next projectile. It is technically NOT a form of auto-loading as it is done manually, but because it is disguised in the trigger pull, it generally falls under self-loading.
Trigger-Operation is simple: Whilst air guns usually use a valve for a trigger to perform only one task, trigger operation allows us to give it another job, or more. In such cases, the trigger can be connected to a floating inner-barrel that moves back and chambers a BB, or it can be used to close off valves as it is pulled whilst another is open.
Trigger operation is a work-around when it comes to building semi-automatic air guns without the usage of a complex gas system.