I have experienced it in the sim. It is violent and uncontrollable unless immediate action is taken. At low altitude it is fatal. The change in attitude is so sudden that if you don't have your wits about you, you forget to push the master disconnect and just pull on the yoke for dear life. I suspect that they may have had a runaway, were taken completely by surprise and didn't know what hit them. If I remember correctly, Stab Trim Runaway did not even have memory items until late 2000's. Can any old CRJ guys remember when this was changed in the QRH?