A new feature in Revit 2011 allows you to ‘lock’ or ‘unlock’ parameterised dimensions in the Family Editor.
In practice, this allows you to instantly flex a family without having to open the Type Properties dialog box.
‘Flexing’ means to test the relationships and parameters in a family by adjusting the dimensions within that family. If something breaks, you know that you have a problem. You should always flex families within the family environment, before unleashing them on a project.