Stress bar gos on top of strut tower bars. You mount it in your engine bay.
In theory it stiffens the body because it reduces body flexing, but providing another piece to bridge the gaping space of engine bay.
In Focus the benefit of the Stress bar is not as significant as in other cars, because our body is already very stiff (especially up front), but people still claim imprved handling.
Overall, for 40 bucks it is a no breainer - makes the car stiffer a little bit, dresses up the engine bay...
oh and it installs in 20 minutes.. the easiest instal out there.