The green "guts" represent the pore walls of a zeolite (AFI) where methane molecules (moving balls) can go in and out - just as they want. To rate how fast the "going-in-and-going-out" proceeds the balls are quickly stopped and those located inside the guts are being given a red color (tracer). Then, the balls are allowed to move again but every time a red ball leaves the pore it looses the red color - for good. As none of the white balls is painted red at any time after the pause, the number of red balls gets less and less as time goes on (red curve in the diagram). The entire process is therefore called tracer exchange. The related article can be found [here].