The magic here is the use of a Proxy field for the field typically called [Related Parent].
For example, if you have a table of Stores and the key field is [Record ID#], but the [Record ID#] is meaningless to the users. You bring down the Store name in a lookup field on the relationship, and then edit the field properties for the [Related Store] record to select Store Name as the Proxy field.
Then on the form, you use [Store Name] and not [Related Store]. In View mode the field on the form will have as [Store Name], but in edit mode it will behave as [Related Store] allowing the user to select the [Record ID#] of the Parent Store record.