It's not a spit plug they still run. It's not a slipped belt because there's plenty of momentum to overpower it.
The only two things I can think of is either one of the wrist pins broke loose and locked it up or it over fueled and hydrolocked then broke an internal part.
Oh one more thing could be a culprit the flex plate broke and can't spin.
The last one is easy to check take off the torque converter dust shield and look in there. Also try turning the flex plate with the proper tool not just a screwdriver.