AFAIK, the dyno jet doesn’t have any load cell to “take a reading” and measure torque.
It uses a formula to calculate the speed differential over time, and calculates how much power it takes to accelerate the drum(which has a known mass) that amount, in that amount of time(not a good description I know).
Basically, the rate of acceleration is what’s used to calculate the power.

If you run two cars with noticeably different power levels, say 100hp and 800hp, through the same rpm range(like 2500-6500), you’d notice the lower power car took much longer to get from 2500-6500.
The only thing controlling the “rate” is the weight of the drums and the output of the motor.
More power accelerates the drum faster.

I’m not sure all dyno jets operate this way, but the one my friends have does.