Cast summon while having an active pet. Demon gets that debuff. Wears off after a five seconds, or when your new demon is summoned. Nothing new. It gives conflicting reports on duration depending on when it's canceled. You can get anywhere from 12 hours to 15 days on the duration.
You used to be able to use DE with a Felguard out, then summon a new pet without the previous one being stunned by it. Not sure if you can anymore. Was fun because you could stun lock people with Intercept for about 5 seconds. Not that you can do anything else, but hey, it's funny.