When you start a task in the workflow, the Workflow Manager locks the entire workflow so another user cannot start the workflow. The Integration Service runs the selected task. It does not run the rest of the workflow.
To run a task using the Workflow Manager, select the task in the Workflow Designer workspace. Right-click the task and choose Start Task.
You can also use menu commands in the Workflow Manager to start a task. In the Navigator, drill down the Workflow node to locate the task. Right-click the task you want to start and choose Start Task.