Can't really inprove on Cupcake's answer, but yeah, it does tend to run down to the back of the nappy, but in my experience it wets from where the pee comes out, all the way to the back, not just the back. I have the nappy on quite tight.
Another thing is your butt can slightly act as a dam when laying on your back, so sometimes I lift myself slightly to let it flow evenly all the way to the back. Of course this also means the waist needs to be tight enough so it doesn't flow right out the top, especially by the concave contour of your spine.
I agree with Cupcake too that to some extent (though mostly when sitting) I can make it go up the front a bit more if I squeeze hard. But the bit right at the top, by my tummy, will stay dry unless I change positions.