It uses the entire sensor and interpolates, unless you tell it otherwise (I think it's labeled "smart teleconverter" or such). Whether that improves high ISO performance or not is something *hotly* debated, but general consensus seems to be it doesn't do jack or, if it does help, it's less than what a half-decent NR program would do with a full-resolution image.
|Owens Valley Milky Way by ed rader|
from Sign, sign, everywhere a sign..
|Break by Hank3152|
from Motion blur
|Camp by T bird|
from A Big Year - birds
|The Maasai Shepherd by cgravel|
from - African Man - (Portrait in Black and White + A Border)