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@Dammit
Power2max (like SRM and Quarq) measure the strain between the spider and the chainring.
Force is applied to the axle via the crank arm on either side, but the P2M simply ascribes a certain % of that force to each side based on crank position- it is estimating the split.
Stages measures the deformation in the left hand crank, then doubles it to get overall power.
So- P2M (and others) measure total power and halves it, Stages measure half the power and doubles it.
The P2M/SRM/Quarq approach is considered better from an accuracy point of view as it measure total power output rather than guessing half of it.