Once you have established the line of action (See Gesture basics #1) you are ready to place the three major ovals of the body: The head, ribcage and pelvis. Noting their locations should take you 5 to 10 seconds.
When viewed from the outside, it's harder to see the underlying skeletal structure, and beginners are often tricked into thinking of the human trunk as being one shape like so:
But in fact, keeping the rib cage and the pelvis separate is critical to accurately recording a pose and allows for a much broader and more convincing range of expression. The rib cage and pelvis can each bend, flex and twist independently of one another, allowing for interesting twists like this:
This image has tilt lines on it, that clearly establish the angle of both torso and pelvis. You may wish to note these down as part of your process of recording the underlying positions of the three ovals. Personally, I find that if I am trying to create a pose with a great deal of energy or tension, clearly defining the angles of the body is essential.
As you can see, what's going on internally influences what we can see externally in a number of ways, both overt and subtle. Most people don't consciously know what they're seeing, but humans are closely attuned to other humans and will be able to detect that something is "wrong" in an image that doesn't pay attention to the details of underlying anatomy, even if they can't put their finger on what.
When a body leans, one side contracts and shortens, while the other side stretches and lengthens. We may see folding of the skin on the shortened side. A "notch" where the rib cage protrudes may become visible on the stretched side. What else can you observe about these underlying structures as a body bends and twists?
As you practice your gesture drawing, spend 5-10 seconds noting down the location and angles of the head, ribcage and pelvis. "Hang" them on the line of action, but keep in mind that they can each twist and bend along that center axis.