A Message Wait activity with the property Allow Interruptions enabled behaves like a listener that waits for a notification to pull out instances from the process flow and process them.
It is not part of the process flow. It defines a flow of its own. When a certain event occurs the instances in the process flow are redirected to the Message Wait event flow.
Generally the notification received by the Message Wait event states which instances should be pulled out from the process flow.
The last flow object in the Message Wait flow must return the instance back to the main process flow. You can do this by setting the value of the predefined variable action to BACK.