Tension is sensed inside the fiber and triggers the signalling that leads to protein synthesis. High tension comes either from a heavy load or from a light load lifted when the muscle is already fatigued, which is why sets of 5 at 85 percent and sets of 20 taken near failure both build muscle.
The practical consequence: proximity to failure is what makes a set count, not the weight on the bar. Three sets of 12 at RPE 9 and three sets of 6 at RPE 9 do similar work; three sets of 12 at RPE 5 do not.
Range matters too, since tension is highest when the muscle is lengthened. Full-range reps and lengthened partials both exploit that.