The difference is in the bushings used.
From Teraflex:
IR Bushings feature an innovative inner sleeve that freely rotates 360 degrees without placing load on the bushing. The internal contact surfaces of the rotating sleeve feature an extremely tough, hardened bearing surface that is factory lubricated and permanently sealed. IR Bushings remove bushing bind during suspension compression whether on the highway or the trail. This results in a noticeable ride improvement. Eliminating rotational stress reduces bushing failure. The free movement of the IR Bushing extends its life span.