Hey @Jnight82,
Do you want your task to continue in the same transaction as when it was executed or do you want your task to send something to the other service in a blocking way and then continue the process instance in a new transaction?
Not sure if you have already seen the blog posts: Demystifying the Asynchronous Flag and Demystifying the Asynchronous Flag (II).
The triggerable is explained in How to use triggerable and here in the documentation.
From my understanding the triggerable Service task is exactly what you are looking for. When you make your service task triggerable, it will execute, but it will not complete. It will wait for a trigger signal from the outside in order to continue the process execution. In a way when you make your service task triggerable it will become a wait state. Keep in mind that this is only for service tasks with a class or delegateExpression JavaDelegates and not for ServiceTasks that use an expression.
Depending on how you are communicating with your other services you might want to look into the Event Registry as well.
Cheers,
Filip