Which of the following best describes the technique of using a straight razor for hair cutting?

Using a straight razor for hair cutting is best described as providing a softer edge. This technique allows the barber to create a more blended and textured finish in the hair due to the way the blade slices through the hair rather than chopping it, which is common with other cutting tools like scissors. The blade's angle and sharpness enable a precise cut that results in subtle variations in length, contributing to a softer overall appearance. This effect is especially beneficial in creating softer lines and layers, which can enhance the overall style and blend of the haircut.

The other options do not encapsulate the nature of straight razor cutting; for instance, a straight razor does not create a blunter edge, nor is it primarily used for just final touches or to increase hair thickness. Instead, its design and cutting technique are specifically aimed at producing softer, more aesthetically pleasing results in hair design.