Here is a closeup of how the Thule 400XT Aero fits onto the Scion xB.
The rack is held in place VERY solidly. Since the aero base feet lock onto the crossbar, the rack cannot move side to side. And a secure downward force is created when the rubber coated metal piece (that fits into the door frame) cinches tight with the support base which sits on the rubber pad (that matches the roof profile).
Note also that the rack can then be locked to the vehicle.
I believe I read somewhere that the maximum load should not exceed 250lbs...which of course must be subtracted then from the measly total cargo limit of 850lbs.