I've got one of those. The idea is that you place the surge tank at the highest point of your whole cooling system, so that all bubbles eventually find their way into it.
The normal radiator cap is replaced by a non-sprung one and your usual rad cap goes on the top of the surge tank. The idea is that air bubbles reduce the cooling efficiency because they reduce the coolant to metal surface area. I think the idea is also that air bubbles are compressible so allow the coolant to boil at a lower temp.
R&S tested one and it apparently worked pretty well with a 4 degree drop in temp or something. On the MX5, you only run a pipe from radiator top to surge tank, top of surge tank to overflow bottle, and bottom of surge tank to bottom radiator hose. The other hoses are blanked off.
So bubbly coolant goes into the surge tank, and it's the non-bubbly coolant from the bottom of the tank that gets fed back into the radiator. The Trust one comes with clear hoses, supposedly because you can see that the coolant coming out of the surge tank is bubble-free