Call External Trigger Output
Take a look at the example workflow called "Call External Trigger" built into the Workflows preferences to see an example in action of how this object can be used to simulate a function / callback.
Why use the "Call External Trigger"?
This has several advantages over the previous way of calling an external trigger using AppleScript.
By not using AppleScript, there is no external dependency on the Alfred app name or version.
Call External Trigger can be configured to call an external trigger on 'self', meaning no dependency on the current workflow bundle id.
Not only can the input argument be passed through to the external trigger, but also all of the current stream variables.
Significantly higher performance, as these calls are made internally and directly as per any other workflow stream connection.