Other than the Double Tap shift lever, perhaps no component in the SRAM road component family represents a greater departure from familiar mechanical territory than the Force Rear Derailleur. It's a bit of a technical story, but it's one whose benefits are evident the first time you run through your gears: You'll experience crisper, more precise shifting than you've ever felt before.
Virtually every rear derailleur you've ever used, pre-Force, was built around 3 springs: The B-position spring, the pivot spring, and the parallelogram spring. What SRAM learned as they developed their rear derailleur, though, was that a 2-spring system built with more powerful springs outperforms the 3-spring standard. By eliminating the B-position spring and fixing your derailleur's B-position in place, the Force derailleur better manages the gap between the guide pulley and your cassette cog. The stout strength of the springs is directly related to the inherent mechanical advantage of the Force Double Tap shifter. Strength requires strength, and it results in punchy, quick shifts.
The other half of the story is SRAM's Exact Actuation technology. Most rear derailleurs pull slightly different amounts of cable depending on which end of the gearing spectrum they're in. You can feel this when the first few cogs of your non-SRAM system shift perfectly, and then the shifts are off. With Exact Actuation, the Force derailleur pulls the same amount of cable (and thereby moves the derailleur the same distance) with every click, no matter what gear you're in. Each click gives you a predictable 3mm of cable pull.
Thanks to the 2-spring design and Exact Actuation, the Force rear derailleur is perhaps the most user-friendly derailleur we've ever seen. SRAM refers to it as "a more tolerant" derailleur, and practically speaking it means that it's easier and quicker to set up, it stays in adjustment longer, and it provides more accurate shifting even if you're riding a bike with thicker-than-normal dropouts, a sluggish freehub body, a worn cassette, or with a bent derailleur hangar.
The B-knuckle is made from aluminum, while the inner link is magnesium, the cage is a carbon/aluminum mix, and the hardware throughout is titanium. It can accommodate cassette gearing as low as a 32t cog, and it has ample chain wrap capacity to handle any SRAM crankset/cassette combo. It has a warm gray finish to match the color of the other aluminum components of the Force gruppo.