The EDITACTION preference setting lets you define the use of an editaction template instead of the standard edit. If EDITACTION is defined as text, then hide the form. If EDITACTION is defined as form hide the normal text area and only edit the form.
When EDITACTION is defined as text or form the Edit and Edit Raw buttons simply add ;action=text or ;action=form to the URL for the edit script. If you have defined an EDITACTION preference setting you can still edit the topic content or the form by removing the ;action=form or ;action=text from the edit URL in the browser and reload.