Freeze your rex using a Cryopod, release it within a cooldown period, you will an unconscious one on the ground. Wait until the Cryo sickness is over, then there comes the most interesting part. Approach the faint rex by its tail until you see the orange roar option, and press it. Your rex will suddenly stand up and roar as usual while immediately return to the ground in faint status again, as if the roar never happened. Such a feature can be fulfilled more than once.
When I was training my Rex a raptor attacked me and I almost died but my Rex came and killed it before i died and also there was a another raptor but when I went in at the back of my Rex the raptor ran away and my Rex chased after it so I was unguarded so the raptor thought he could just run and jump on me and kill me but my Rex got unstuck out of the stone and turned and ran and when the raptor got its first damage on me it got Rex.