jockeys have to wear flak jackets now and theyv’e made a tremendous difference...they really absorb the the energy of violent hits..
Most commotio cordis( if that is what happened) events occur in baseball...from the baseball hitting the spot during cardiac repolarization...supposedly occurs with chest protectors also..from what I have read...young people have weaker thorax and cannot absorb a strong blow. Commotio Cordis can occur to car drivers sternal area hitting steering wheels in crash.. hence air bags in steering wheels and 3 point seat belts.
Good review of commotio cordis in Wikipedia.
Pray player survives.