When we do a bending moment diagram of a continuous slab or beam, we will observe that the max moment occurs at mid-span and over the supports but in the opposite sense.
At mid span tension is at the bottom and over the span tension is at the top. In between there is a region of zero or low bending moment. This region is near the third of the span. Hence, if a lap is needed, this region is suitable. Hope you could understand why is reinforcement lapped 1/3 of the span away from the support in beams and slabs?