Nothing but pure upper body power. This dude could go on and break the all time record.
Monster Persian/Kurdish genetics a la squadfather
Dude looks like he weighs 680 pounds himself. Bill Kazmaier benched 661.4 pounds RAW at 330-340 pounds in 1981. THAT's true strength. Here he is benching 633 RAW at about 320-330 pounds bodyweight with NO spotters. Had he not torn his pec, tricep and lat later in 1981, he would have easily smashed 700 pounds.