Transform Matrix Math help

How exactly does the movieclip.transform.matrix.rotate work?

I mean, if the movieclip is at 0,0, the movieclip.transform.matrix.rotate will rotate the movieclip with 0,0 as it’s pivot… but i dont get it’s behavior if the movieclip is not at 0,0. for example:

var example_mc = new MovieClip();
addChild(example_mc);
example_mc.x = 275;
example_mc.y = 200;

var mtrx = example_mc.transform.matrix;
mtrx.rotate(deg2rad(90));
example_mc.transform.matrix = mtrx;

(deg2rad is a function that converts degrees to radians)

after this code, example_mc.x becomes -200, and example_mc.y becomes 275. Can you explain (with some math perhaps) how the movieclip was rotated? Apparently… the movieclip isn’t being rotated at the point (275,200)…