Weighing in at only 112g (54t), the solid aero ARC allows you to remove the front derailleur for even further aerodynamic gains due to the special tooth design to retain the chain. The ARC's subtle shape is designed to mimic the firing patterns of your leg muscles whilst riding, increasing the gearing size when you most need it.
Due to the higher speeds involved in a time trial, inertia and momentum is much greater than normal. This causes pedal speed to be much smoother, and means that for a time trial (high speed) large increases in gearing and dramatic noncircular shapes are not optimal:
The ARC increases gearing quickly at the beginning of the pedal stroke as this is when the main force producers activate (gluteus maximus, vastus medialis & lateralis, as well as the tibialis anterior). After this period gearing is maintained all the way through the highest torque phase before gradually decreasing down to a minimum gearing at bottom dead centre. This smooth dropoff is when the effect of gravity and the lower leg muscles come into play.
The unique time trial specific design will help increase power output by 3-5w, and allow a smoother pedal stroke than normal. Unlike other noncircular designs you can train on the ARC ring without causing a detriment to your pedal stroke and swap between round/ARC rings without difficulty. In addition, there will be little to no power inflation (<1%) when using the ARC on a crank/pedal based power meter.
ARC 54t & 58t chainrings are designed for the 40-55kph speeds involved in time trialling and fast triathlon riding. These are high inertia scenarios, where pedal speed is much more consistent than situations involving low inertia (such as climbing). Therefore the ARC carbon rings are optimised with a subtle noncircular shape.