Basically it's "slingshotting" your jig, way back underneath a dock, to reach the heavily shaded waters where Crappie tend to hang out. You use a spinning or spincast outfit, let out line equal to about 1/2 - 2/3 the length of your rod, grab the jig by the head (with hook pointing away from your fingers) and pull back to "load" your rod ... then aim the jig to skip across the water, starting at the dock edge or as far under the dock as water level will allow ... release the jig, and once it passes the end of the rod you release the line coming off the reel.
It's just a form of casting, that allows you to place your bait back under a dock, pier/walkway, or pontoon boat, when any other casting method is not applicable. It takes a little practice, but it's a very effective presentation. It's easier to do on stationary docks (docks on poles), but even floating docks have openings that can be "shot" into. The shooting technique can also be used to cast a bait under low hanging obstacles, when a sidearm or overhand cast are difficult to manage.
Everybody that "dock shoots" has their preferences for what equipment to use, and how they perform the technique. Search "dock shooting" or "shooting docks", and you'll likely find many of the posts/threads discussing the technique, as well as many videos showing it being done.
... cp