When you disable a field, the field (and any fields nested within it) is not displayed in the page, and its value expressions are not evaluated. If the view already contains a value for the disabled field, the value will not be modified.
Keep in mind that global.* attributes are derived from enabled fields only. If a form dynamically disables a field (instead of hiding it), this field value will not be available through the global.* attributes.
<Disable> <eq><ref>userExists</ref><s>true</s></eq> </Disable>
Disable expressions are evaluated more frequently than other types of expression. For this reason, keep any Disable expression relatively simple. Do not call a Java class that performs an expensive computation, such as a database lookup.