The tip is a little more complex than your setup, sorry it wasn't clear enough. Here is a project showing it:
http://n4te.com/x/8519-legs.spine
Even though the hip is not a child of the torso, move the torso and the hip moves with it. Click the transform constraint and set the X and Y mixes to zero. Now when you move the torso the hip does not move with it.
I have the legs as children of the hip. If you don't have a hip, you could change the transform constraint bones to the two thigh bones. When you do this you'll find the 2 bones move to the hip bone position. This is because the constraint's offset X and Y is set to the position of the hip bone in relation to the target bone (the torso). Set the mixes to 0 so the legs move back to the unconstrained pose. Now click Match to set the offset to the first constrained bone. Set the X and Y mixes back to 100 and you'll see the bones are now both at one of the leg bones. If you need to position the constrained bones differently, use a hip bone.
For more of your understanding, delete the transform constraint, select the hip, New > transform constraint, choose the torso. Set the X and Y mixes to 100 and see what happens -- the bones move, but we probably don't want them to. Set them back to 0, click Match, and set them to 100 again. You'll see nothing happens because the transform constraint moves the hip bone to the torso bone plus the offset, and you've set the offset to the difference between the torso and hip bone. You're done, the constraint is working, move the torso to see the hip move.
