If you click on the link in Denny's post he has a pic and a little write up on it.
The Free spin kit gets rid of the sealed/nonservicable unit bearing/wheel hub assembly. Since they are sealed you can't pull them apart and repack the bearings. When one or more of the bearings dry out the assembly will fail and you will get play in the wheel hub assemlby, grinding noise, etc.. In extreme cases of failure the bearing and seize itself to the axle shaft, and take out a few various other parts with it (rotor, caliper, etc..). When the unit bearing/wheel hub assemlby fails it's a throw away piece and must be replaced with a new one.
The Free Spin kits gives you heavier duty outer axle shafts, a fixed spindle and a new wheel hub with serviceable bearings. This way you can periodically pull the assembly apart and repack the bearings. They also come with Warn Premium locking hubs.
I haven't seen a 05-07 unit bearing cut apart yet, but on the 99-04 trucks both bearings are inboard of the wheel mounting flange and almost placed back to back. The Dynatrac kit for those trucks did a better job spacing the inner and outer wheel bearings to better distribute the load on the bearings. This may be a benefit of the 05-07 kits as well.
So as a opposed to spending $300+ on a unit bearing you spend a few bucks on some grease and a wheel hub seal everytime you repack the bearings.
Lifted trucks with larger/heavier tire and wheel combos stress the unit bearings more and accelerate the wear on them so this is especially a nice product for all the lifted 05-07s that want a heavy duty upgrade that they can service themselves and don't have to wait for it to fail to replace it.
