???
Most smart trainers AFAIK measure (really, estimate) power based on measured RPMs (flywheel speed) versus assumed torque (which I assume is based on a predetermined value for a given amount of power applied to the electromagnets in the resistance unit) plus/minus a calibration factor that corrects for the drag in the system (unless you have a Tacx Neo, which in theory doesn’t need to be calibrated because it doesn’t have a belt drive).
Most smart trainers don’t directly measure torque/power like power meters do (RPM calculated via an accelerometer or cadence sensor vs torque calculated based on mechanical strain).
My old Taxc Flux measured something like 30 watts high at threshold because (I assume) the pulley warped after the belt slipped off of it.