Brembo is correct, a magnet is just not strong enough to hold that much weight out that far.
It looks like the rig is something like this (keeping with the crappy paint images...)
To me it seems like that is what is going on, and it seems like it is perhaps bolted to the frame or something like that with some U bolts.
It seems to me that movies would use this type of setup, as it probobly requires some work to the car's underbody, like taking off the skidtrays and stuff. It seems like car magazines and such wouldnt use this type of set-up, or if they do they must have the car for some time.