I'd like to know more about the Field Name and Action, as well.
If you're looking to send an email to the person whose email address was pulled into the custom field, though, I can only think of a way to do this somewhat manually:
You can create a Smart List of everyone whose CUSTOM FIELD has been changed; export the list; delete everything but the CUSTOM FIELD; change the header of the CUSTOM FIELD column to "Email Address"; then import that new list as leads to a Static List. Make sure to block new leads and updates so you a) don't create duplicate fields and b) add anyone to the list who may already be in your database.
Then in the background, you can have a campaign that says, "When someone is added to STATIC LIST", send them this email.
Not totally automated, but not too horrific.
You can even set up an alert when that field has been changed so you know when the export the list (or if you even have to do so on a regular basis).
I hope that makes sense!