heres what TDT has to say about the IC
Survivor: Cook Islands episode 3 detailed IC spoilers/ speculation
This one was lifted directly from the far-more popular Palau season: "Hot Pursuit" (which was also in Ep3, and for immunity). Each tribe, chained together, runs in shallow water around a large circle, with backpacks filled with sand (or something else of similar weight). As players drop out, they have to hand over their bags to their remaining tribemates. Eventually, one tribe catches the other, and wins.
The only real question is: Is this for reward, immunity, or both? Our guess is the latter. Although it should be pointed out that the new challenge arrival pics seem to show most of the contestants wearing different clothes than they do in the challenge (Sundra, in particular, who wears more clothes to actually compete in the challenge, for some reason). So there could actually be two challenges.
As for this particular challenge, from the looks of it, Rarotonga seems to have a fairly easy time catching up to the older and weaker Aitutaki. JP and Adam (now together on Rarotonga) are seen tackling Yul and Cao Boi (now on Aitutaki), with Yul laden with multiple bags at the time. Someone on Aitutaki is also shown throwing his bag down, apparently in frustration. Rarotonga wins? http://www.truedorktimes.com/survivor_cook_islands/spoilers/ep3/ic.htm