Engine Torque & Motor Mounts
The amount of engine "flex" (twisting, as it's called here in this thread) depends on how much torque the engine produces & at what RPM. Under a "brake torquing" load, the engine can "flex" quite abit, especailly a high torquing motor. Continued brake torquing can weaken mounts quickly, just as it can if the car "hooks" up real good when "launching" from start & shifting through the gears (manual). That's why a true racer will have "Line-Lock" &/or a "Trans-Brake" to lessen the chance of breaking a mount. Also, as stated in this thread, you can do a number of things to lessen the chances of "ripping" the motor off the frame & outta the engine compartment. One is through solid mounts, & an engine flex plate can greatly help, but the ride can be terrible. What us "poor folk" (namely me) have done in the past is to use the heaviest mounts available & use a couple of short pieces of chain as a safety feature. the chains are slightly loose to allow for the motor to flex under torque & if a mount should break, the chain will act as a safety to prevent the motor from falling out of over flexing (the motor will only go so far before the chain will stop it),
Dave