It's all related to the (y) vs (x) coordinate system. What you have to imagine is a circle with an origin (0,0) and which has a radius of (1).
At 0 degrees around this circle you will find the coordinates (1,0)=(x,y).
At 90 degrees around the circle you will find the coordinates (0,1)=(x,y).
At 180 degrees around the circle you will find the coordinates (-1,0)=(x,y).
At 270 degrees around the circle you will find the coordinates (0,-1)=(x,y).
If you go 360 degrees around the circle you will find the coordinates (1,0)=(x,y) again.
Now sin (ω) = O / H. If you go -270 degrees around this circle you will find the coordinates (0,1).
It turns out that the opposite side of this right angled triangle (used around the trigonometric unit circle) which has been squeezed into a line - is the (y) coordinate, and the hypotenuse is actually equal to 1. (y) is in fact equal to 1 itself.
sin (-270) = O / H = y/1 = 1/1 = 1
