Swiftwater Rescue is a very dynamic environment. A great example is unpinning a boat, once the boat is freed it will travel downstream very quickly and with a great deal of force. This is why we typically add a second independent tag line. Now lets consider the effect on a multipoint anchor. Life is great when you can pull in a straight line and the load is shared equally amongst the anchor points. What happens to a load sharing system when the object moves? One of the anchors will need to take pretty much all of the load. Since we typically choose a multipoint system due to lack of a decent bomb proof anchor, our system is likely to fail.
A self equalizing anchor system is more flexible. As the towed object moves, the angles change to ensure all anchor points share the load. Eventually, even this system will fail when the angle is two wide as in a vector pull. This is where the extra tag line comes into play.