ICS3 (Internal Cable Stops) erradicates the need to run internal cable housing the entire length of the tube, which adds friction and up to 200 grams of dead weight.
Internal Cable Stops on the other hand, function much like external stops. The housing fits into the stops and the bare cables travel in unobstructed paths through the frame, into the bottom bracket cable guide, and straight to the derailleurs.
ICS3 also reduces aerodynamic drag by placing the cable housing entry out of the wind behind the stem. In order to have the cables enter the top tube, yet travel unobstructed through the downtube, Cervelo's ICS3 frames use Teflon-lined stainless steel tubes, curved to guide the cables into perfect alignment with the Bottom Bracket cable guide.