Since you specified partial water changes and not top-offs, then it makes a difference. With water changes, you are actually removing the water from the tank, therefore, the water you replace needs to be mixed with salt. Unfortunately, when making your salt mixture, salt does not dissolve immediately. So when you prepare water, you need to add salt, and use a pump/powerhead to continiously stir water around. Then wait a day or 2 to verify you have the correct salinity using a hygrometer.
If you were just topping off water due to water evaporation, then all you need to do is just add water (not salt mixture), since only the water evaporates, and not the salt.
I would have to assume that if a pro came in to do it, he would already have a container of salt water already mixed at the site. If they wanted to do an "on the spot mix" they could probably just keep adding salt into the water and only measure the salinity for the salt that has already dissolved. When they finished, they would just dump the water and undissolved salt down the drain. Though kind of haphazardous, it is feasible. Just my assumptions though.