Actually, I think that the "Swedish" tether will reduce downward rotation as well. Here's why:
When I install a rear facing Marathon without tethering it, I have to leave the seatbelt "loose" enough to maintain the proper recline. However, when I use the tether, I can pull the seatbelt significantly tighter, then use the tether to counteract it. This reduces the amount of seatbelt (or LATCH strap) that can stretch during deceleration, and should reduce the downward rotation.
Think about it this way, if you install the seat without the tether, then push down on the back of the seat, it rotates downward quite a bit. If you install the seat at approximately the same angle with a taut tether, then push down on the seat, it may still move, but it will rotate significantly less.