Jeff,
I was comparing the billies and the others.
The 5100's have oil where the piston moves inside of the shock. Then there is a "floating piston" or really just think of it as a wall to keep the nitrogen pressure on the other side of that wall. This keeps the oil as oil so it doesn't get aerated.
The other types of shocks (dampers if we want to be correct) are emulsion and have oil and nitrogen in there, mixed. If you move an emulsion shock up and down two or ten times the valving feels one way. If you do it 20 or 30 times the oil and nitrogen have turned into a frothy foam that has a completely different valving property. This new foamy mix moves through the piston (which controls the valving) MUCH easier than pure oil so your valving goes from what you feel driving down the road to a much more mushy feel that at some point will almost be non existent in a resistance sense. Then you are riding on the coils basically. (as long as you dont hit any big bumps some people love this, picture an old cadillac going down the highway and bouncing along, its a nice ride until you hit the big freeway bump and cant stop shaking)
http://www.calsci.com/motorcycleinfo/Images/SHOCKS5.gif
The center pic is of an internal reservoir shock. It works the exact same as an external. The only difference is the volume of oil.
Another good example
http://www.tonyfoale.com/Articles/Springs/Spring2.gif
These diagrams show the simplest technology of a shock. The ranchos, Old man emu's etc lack the piston. They are simply filled with a prescribed amount of oil and then pressurized.
The idea of the front coilovers having an external reservoir helping them is:
An external reservoir would increase oil volume BUT that oil volume would not be increased enough (unless you used a LARGE reservoir) to overcome from the increased heat that the oil traveling through right angles and a thin hose would create.
The other benefit of external res is increased shock travel with the same body length when compared to a internal res shock. (Just look at the diagram, when you put the res somewhere other than internally it saves some room thus letting you put a longer shaft into the shock body) Hence what fox calls its short bodies. (An external res shock that takes advantage of the extra room with a longer shaft)
This is worthless for the tacoma/4runner suspension since our cv's can barely handle the stock travel let alone a 3 inch increase. 1 inch extra is pushing it.