The 5th line, when properly tuned, should have the same tension as the front lines. It is true, if the 5th line is too snug, your kite will want to start sinking from the sky (too much pull on the leading edge). You can see this if you pull on the leash while flying the kite. If you pull about 6 in, the kite will start sinking, if you pull about a foot, the kite will do a dive(hindenburg). If you get enough wraps on the 5th line, your 5th line is effectively shortened and you will start seeing the above effects. This is good to know and a benefit in overpowered conditions. If you are fully sheeted and still overpowered, you can pull on the 5th line to spill additional wind.
Once you start riding in waves, the flaw in the 5th line is readily apparant in terms of strength. If you're kite is caught in a wave, that is a pretty significant force, pushing the kite, while you act as an anchor treading in the water.
As far as the kite sinking when the 5th line is pulled, this is the same concept as to why the windwing SAFE bridle works. The rapture doesn't have a bridle, but has shortened tips (chop top), an ultra stiff leading, and a flatter shape (less U) to the kite. That is how the rapture is still compatible with the SAFE system. Think of it as chopping the tips off of a rage and having the rage bridle being the front pigtail for the kite.