I think David is using the word "dynamic" in it's ordinary usage, and not as a term of art. The definition of "dynamics" is "a branch of mechanics that deals with forces and their relation primarily to the motion but sometimes also to the equilibrium of bodies." Adjusting something dynamically means that you are adjusting it while it's in motion, as opposed to when it is static. When you balance an automobile tire statically, you find where it's center of gravity should be, and then add weights to it until it appears to be balanced. When you balance a tire dynamically, you spin it, and add weights until it spins smoothly, without vibrating.
Using that definition, dynamic tuning just means tuning the rig while it is in motion and under load. Tuning the rig at the dock is, to a certain extent, educated guesswork. From experience, the person tuning the rig knows the mast should generally be erect and symmetrical, with a certain amount of tension on the stays and a certain amount of rake and other characteristics, but you don't know for sure how it will behave until you sail it, with the rig in motion and under load. I always tune my rig in the slip first, and then sail it and make sure that it behaves the way it should when the boat is in motion and the rig is under load.
I think most sailors make too much of rig tuning. For the average sailor (casual cruiser and occasional beer can racer), it's enough if the rig is straight and erect, with enough tension on all the stays to prevent the rig from moving around in a seaway, and with enough rake to produce a comfortable amount of weather helm. You can find lots of books and other sources with clear instructions on how to achieve those basic objectives, and most people don't need to pay someone $100. an hour to do it, if they just don't allow themselves to be intimidated by the thought of it. Fine tuning the rig for optimal performance in light air or big winds requires a little more knowledge, but it's the kind of thing most people can learn with experience and a little reading.
If your first thought is to read about how to do it and then try it yourself, the likelihood is that you'll achieve a satisfactory result, and never again be intimidated by the thought of it. If, however, your first thought is to hire someone to do it for you, you'll never get beyond the mystery of it.
If you have a trailerable boat, and have to rely on a pro to tune your rig, you'll be reluctant to trailer it anywhere, because you'll have to hire someone to rig it for you when you get there, and then hire someone to re-rig it when you get it home. It's well worth the effort to learn how to do it yourself.