-- Last Updated: Jan-03-13 4:11 PM EST --
As you can see in this picture (also linked above), I tie the nose of one boat to the tow ring on the front bumper.
The carriers and cross bars make a very rigid unit - the front tie-down keeps the nose from bouncing up and down. Only one tie is needed as there is no side to side wiggling at all.
I use bungees as a quick tie-down, but also NRS cam straps at each end of the boats, as you can see in the picture. The bungees are nice in that they secure the boat quickly on windy days, and if you're just driving away from a ramp or some other short distance, they suffice. They're furnished with the rack, but there's no rule that says you can't supplement them with straps.
A surprising number of people get hung up on this aspect --- sure, the makers of Kayakpro and Goodboy depend on the bungees alone, which are quite beefy, but I think many like me add straps. It's certainly no reason to doubt the usefulness of the rack itself.