To really understand this, you have to understand the concept of thermal relaxation time. This is a theoretical concept that explains the speed at which a structure in the skin loses heat to the surrounding structures. The idea is that the large an object the slower it loses heat, while the smaller an object, the quicker it loses heat. The TRT is based on the time it takes for half of the heat to be lost to the surrounding tissue.
The evidence, based on theoretical models that have been tested in vitro (meaning bench research, not in living people) analysis, is that an individual skin cell has a TRT of about 1-2 ms. Coarse hair seems to have a TRT of around 30-100 ms depending on how coarse it is. Fine hair is somewhere around 3-5 ms depending on how fine it is.
When the laser pulse is on, you are pushing energy into the structure (hair and skin). At the same time the structure is losing heat (sort of like a funnel). So theoretically, if the time that the heat being pushed into the skin is longer than the TRT of the skin, then it is less likely that you will burn the skin. This is why a variable pulse laser is safer. You can treat at a longer pulse width and help protect the skin. This is also the reason why Candela, which made the decision to use a fixed pulse at 3 ms had to go with cryogen cooling. The cryogen helps to reduce the skin temperature and decreases the TRT for the skin significantly.
The problem for variable pulse has to do with the hair. You can always put the energy into hair quicker than the TRT, so it is is possible to treat coarse hair with a short pulsewidth. The problem is that you can’t treat fine hair (which has a short TRT) with a long pulse because what happens is that the hair loses the heat faster than you are putting it in.
So treating coarse hair doesn’t really matter. Treating very fine hair does. And when it comes to treating fine hair the difference between 3 ms and 5 ms is enough to have a impact on very very fine hair.