Look carefully at the whole system
I think you are asking about the tension on the control lines. If you take a close look at the bottom of the control lines, you will see some fabric "pockets" which attach to the lines. When the sail drops, the weight of the sail pushes down on those points and that puts tension on the control line. Likewise, when you raise the sail, the control line tension is relieved so that the sail can fill without being restricted by the control lines. Properly set up, the topping lift does not require any adjustment for raising or lowering the sail. I have noticed that sometimes I do have to help the flaking a little for it to work smoothly.The system does require some infrequent but careful adjustment to work properly. If the lines are not vertical (or should I say parallel to the mast), they can cause the sail slides to jam and it becomes extremely difficult to raise the sail. With a full batten sail and misadjusted control lines, the forward pressure on the sail causes the slides nearby to twist and that creates a lot of friction.