Implicit iteration occurs when connecting services with different depths, creating a new list of the outputs of the service.
Services output ports have a defined depth, just like service and workflow inputs.
If a service expects a list, but you provide it with a single value (like from the [string constant|taverna:String constant]the Text Constant), Taverna will simply wrap the value in single-item list(s) for you. Be careful, though, if you later change the workflow to do an implicit iteration over the service outputting a single value, the following service would receive the full iteration output in one go.
To workaround this, you can write a Beanshell script taking a single value, and returning a list with a single item.
This will force iteration over lists of lists below in the service below expecting lists.