In a nutshell, Stages Power Meters are mounted to the inside of left crankarms, and translate stress on the material as a result of your pedal stroke into power figures. Up until now the company has only offered meters on alloy cranks. Why? Because the conventional strain gauges typically housed in the plastic Stages shell don't work on carbon. Carbon can be an odd (and also wonderful) material because it isn't consistent like metal - it's anisotropic, meaning it has properties that differ according to the direction of the measurement. Fiber orientation, the interaction of the weave, etc, all come into play. It's complicated. Especially when you throw varying temperature into the mix as it changes how much the material deflects under a given load.
So how did Stages modify their power meter to work on carbon cranks? That isn't a secret they're willing to share at this time, and rightfully so. It took several years to figure out how to do it, and requires the use of an entirely new strain gauge. The end result is a meter with automatic temperature compensation.
According to Stages, "There were many challenges, including: the nature of the composite materials; fatigue characteristics; bonding to a composite material; effect of temperature changes; direction of forces and strain. To build a carbon Stages power meter, Stages engineers spent years in development and testing specifically to achieve the same consistent and accurate power measurement across all riding conditions that were introduced with the original Stages power meter.”
You won't find a Stages meter on a carbon SRAM crank (perhaps as a result of SRAM owning Quarq power meters), however Stages developed its own crankarm with FSA that will work with SRAM MTB cranks when paired with an aftermarket Stages spindle. It's even shaped similar to SRAM's carbon cranks.
The meter housing has also been updated, and now measures 5mm versus 8mm, as well as a sleeker overall look. The battery door is now more water tight than ever, and thanks to a new fastening mechanism it's also stronger.
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