Sync to Marketo field: What next?

Conditional formatting:

Several discussions refer to a workflow when blocking leads from syncing using a Sync to Marketo field. How is this done? Is it in Salesforce? Is it a workflow? What does it look like? Could someone elaborate how this system works once the boolean field is created?

Thanks in advance

Bill

Reference:

Using Conditional Syncing between Marketo and Salesforce.com