So say a pendulum consisting of a non rigid string and mass bob is swung above the horizontal position. It's given an impulse at the bottom so that in the swing, there is nothing but tension and gravity acting. At above the horizontal, the gravity is pointing down while the tension can only point radially inwards. There is nothing causing the rope to have tension then at this point. Gravity is pulling down. So what is pulling radially outwards to oppose the rope? So why is there tension? at below the horizontal, there is tension because the weight is directly pulling on the rope. this makes sense. But not above the horizontal.