The trajectory of a pose is set up in the first minute! There's something that happens as we wave the pen around the page waiting for it to contact. We are aiming! Eventually we feel comfortable putting a line down and hope that its the right one. In the onset of the drawing those lines need to have a sense of directon aiming for a target! Imagine if a there existed a magic bullet that when fired it didn't just go straight, it wiggled around and drew a line that formed our drawings. The same kind of aim a marksman may have when he fires a bullet to a target is the same kind of aim we need when we fire off a sketch, but our bullet moves around in that magical line. All too often we don't take the moment to aim and we start putting lines down that mean nothing or fight against each other on the way to that target.
Those first few structure lines are the most important because those are the ones that we are building on and they act as your scope for the rest of the drawing to follow. So all the best as you aim and fire, drawing out lines that are purposeful and hopefully.........hit a bullseye!