This is how I see it. The reciprocating mass internal to the engine tends to press down on the passenger mount when the car is sitting still and engine reved. The motion is compounded by an order of magnitude in forward when the motor/trans is trying to spin the driveshaft and all behind it. Only when there is an oposing load does the motor build its 300, 400, 500 ft-lbs of torque really twisting the motor. However, when in reverse, spinning the driveshaft in the opposite direction easily overpowers the internal engine torque and tends to rotate the engine in the oposite direction, compressing the drivers mount. You don't see engine torque straps on the passenger side of race cars because the serious torque is there only in forward. Years ago my wifes Oldsmobile broke a mount. Never knew it when driving until one day when changing the oil, I backed the car up with the hood up and saw the motor raise about 3 inches on one side. Putting it in forward compressed the bad mount and the motor didn't move.
camachinist, you out there?
1966 Shelby GT350 6S383 Paxton Blower and T5Z
1968.5 R-Code FB, Zero option, 4-speed 4.30 rear, radio delete car
2006 Viper Coupe w/ Paxton
2010 Custom Harley Street Glide