>...but I'm not sure how I 'activate' the trigger [if I'm even asking this correctly] such that when I do an UPDATE on a reason code, the trigger automatically fires.
>

You define a trigger for a specifice table. If you define your trigger as an insert and update trigger, whenever a user inserts or updates a record in the table, the trigger will fire. Here is the basic syntax of a SQL Server trigger:

CREATE TRIGGER [TRIGGER NAME] ON [dbo]
FOR INSERT, UPDATE, DELETE
AS