Tic Tacs are commonly used by freestylers as a method of self-propulsion and is also used for turning since freestyle trucks are usually adjusted too tight for carving turns. The basic concept goes like this; The skater pushes down on the tail, to raise the front wheels off the ground, while simultaneously moving the nose to the left or right 45 degrees or so to create some momentum, then set the front wheels down. This motion should propel you forward a bit. Then repeat the same movement in the opposite direction. When doing this back and forth repeatedly you can gain more and more speed without even putting your feet on the ground.
The Tic Tac is very important for self-propulsion in freestyle skateboarding. In the olden days, skaters would be deducted points in a contest run if the skater's feet ever touched the ground.