It works by stopping the radiant heat from continuing on into the
attic and warming it up more. Ever sit under one of those radiant
heaters that are used on patios during cool weather? The radiant
heat warms you. Imagine putting a thin radiant blocking barrier
between the heat source and you. You would no longer be warmed by it.

If its stapled to the rafters, which is where it's usually deployed,
there is air flow from soffits up to a ridge vent, taking the heat
between the barrier and the roof out.





Operative word is air flow, do not lap courses, 1/8" space between, do not
close off any rafter cavities.