A quick question to ease my thick skull!

I fitted my rear strut brace a while ago and this thread had me pondering!

If the back one goes on easily when the car is sat on its wheels and it is just a solid bar, what difference would jacking it up make?

I fully understand that taking the load off wheels could potentially move the turrets in which case, if it moves them that much, surely the brace wouldn't fit!

(SENSIBLE explanations please!)