Fourth part (if excluding facial proportions) in my tutorial series. Probably my favorite thing about drawing.
Dynamism is what makes the drawings so lively. Even a drawing without any real action should be dynamic. Dynamism makes your drawings look more natural and interesting to look at. It can also make up for many mistakes and even make them look intentional and part of the drawing.
Line of action is the manifestation of dynamism. It's a sort of invisible line that (I believe) everyone can see subconsciously. I find that there are two types of line of actions: primary and secondary line of actions.
The primary line of action, like the name suggests, is the most important thing in your character. It gives the backbone to the whole pose. That's why it's really important that it's found very early in your sketches. Without this line, your poses will end up plain, boring and unjustified. The secondary line of action is more like a compositional guideline. It helps to justify the positioning of the sub-elements of the drawing, such as clothing, accessories, hair, limbs and shapes.
Regardless the role of the action line, it's main purpose is to add feel of flow in your drawings. This flow is then perceived as dynamic drawing. A good line of action is long, strong and as continuous as possible. These are prerequisites for illustrating dynamic action.