I can't give you a scientific mechanism for it working, but it does seem to help. I'm not sure that it has no detergents in it - considering ATF goes through an auto trans, picking up particles of clutch material and taking them through the filter.
On the injector side, it may be because it's thicker than diesel - simply making the IP work a little harder, creating higher pop pressures and (hopefully) pulling out some crud with it.
I'm not quite sure. I do know that it is a bit more work to shove through an injector(I use ATF in my pop-tester), and seems to lube everything up real nicely.
The other part of the recommended cleaning is also important, though - that of taking the truck out and /working/ it as hard as you can for a while.
Seriously - the old 'italian tune up' is alive and well. Personally, I find that the engine doesn't get nearly hot enough just driving around empty; you'll get plenty of not-burned-off deposits on the exhaust manifolds.
It's only when you hook a trailer to it and give it a good workout that things start cleaning up and burning off.