Its related to the amount of deflection allowed by the poly mounts compared to the amount allowed by rubber. If the engine mounts are rubber, which is softer than polyeurethane, and the tranny mount is poly, then the engine can twist to a greater degree than the end of the tranny. The testimonials you see here relate to cracking of the bellhousing area.
Think of this like twisting a flat piece of metal between your hands. Under a quick snap of the throttle on a powerful engine, the engine/tranny combo tries to rotate, if the engine can rotate quicker than the end of the tranny, then the assembly tries to twist in the middle, just like the sheet would do in your hands. The reason that metal may give before poly has to do with the distance between the mounts and the middle. The distance acts just like a lever to multiply the force applied.
If the engine and tranny mounts are made from the same material, then the whole assembly has balanced forces acting on both ends, and doesn't try to twist.
Hopefully, I didn't confuse you, its been a year or two since I took Dynamics.