We'll we don't. That is something that was done in the past, but is no longer necessary. With all the exotic thermally activated wires we have at our disposal now, tightening the braces or wires is not done.
Thermal wire is dead soft when cold and gets stiffer at body temperature or above.
So, we place it in the brackets when it's cold and it gradually stiffens at mouth temperature producing progressively higher forces and the pressure felt by the patient. We also progressively use larger or stiffer wires which may feel tighter for a couple of hours, but we don't physically or actively tighten them. These are not your dad's or mother's kind of braces. Below are examples of 'archwires' which reflect the shape of the dental 'arch'. These are the thermal archwires which are soft when placed in the braces and stiffen when they reach body temperature to begin the moving process.