The key W and Z are saying “drop chaff” - “drop flares” in the keyfiles, but that is not what is done at all. They are actually doing the same thing : dropping a countermeasure program.
To know more about this : go to your install folder -> Docs -> Falcon BMS manuals -> BMS - Dash 1, then go to page 30 for a full review of the counter measure system.
In the real jet, the pilot use a hat on the stick, the CMS hat, to operate counter measures : look up page 50 and following, for the exact use of the CMS hat.
To set up your DTC, go in a Tactical Engagment and find the button on the right of the map. It will stay in use, even for Instant action and Dogfight module.