Like 90dpscoupe said, subframe connectors connect the front and rear subframes. Other than the subframes underneath, there is like a 3 foot gap of just floor. Basically the front and rear of the car are tied together through the roof and pillars being the strongest structural supports, convertibles having door strengthening and light duty subframe connectors/k-member braces on some. Unfortunately, this is barely adequate even for stock mustangs, let alone mustangs with high horsepower or cars that do autocross/road racing.
How much of a differance does it make:
On a drag racing car, a twisting chassis will cause usually the left rear tire to lift up/get less traction that the right resulting in not only less traction, but wasted energy that could've been planted to the tires (if you see a picture of a race mustang with one wheel 2 inches off the ground and the other 8 inches you'll get the idea).
In a road race/autocross car or even a stock car, imagine taking a really hard corner and the rear end coming around or hopping violently because one of the tires lifted up and lost traction because of chassis flex.
Basically it's cheap insurance against permanently twisting your frame or even your personal safety.
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'84 Mustang 5.0 LX
My car