Plants have auxins (growth hormones) that allow them to "see" light through their growth tips. Cannabis will always turn to the strongest source of light. In a totally light deprived room with a candle at one end, they will stretch towards the candle before dying, searching for more light.
Applying LST is using this to your advantage, it is the whole point of the method. Growth tips closest to the light will grow stronger (apical dominance) and turn towards it. Bend that branch down, and the next closest will produce more auxins and grow more strongly towards the light. The bent down branch will quickly turn back towards the light and keep growing as well, so growth can be controlled and spread out evenly among branches. I hope this helps.