"Marks are your words, drawn me a poem."
The exercise is extremely challenging. There are so many things to think about. Proportion, movement, rhythm, volume, contrast, value, expression. The short amount of time forces the artist to make decisions very quickly. The first marks the artist makes on the page are the most difficult - the instant of creation when the drawing leaps from the mind into the world.
A big blank sheet of paper and a two minute ticking clock can be intimidating. But you have to start drawing somewhere. For beginners, I recommend just going for it. Throw all caution to the wind, look at the model and start scribbling as fast as you can. Don't worry about being perfect. There is no time to measure. Draw large and fill the page. Express yourself.
No matter where you start, your eyes and hand better be moving the whole time, or the two minutes will slip away. Gesture drawing is about deciding quickly and reacting to the model. That's the point of drawing the figure in such a short time, the artist is forced to reduce their drawing to the least amount of elements needed to convey their reaction to the pose.
I draw the basic shadows using the wide, flat edges of the graphite stick, creating a nice wide stroke. The harder you press the darker it gets. To get a nice velvety texture, put a thick pad of paper under your drawing, so when you press down, the resistance is gradual and even, allowing a wide range of tones.
Switching to the corner of the stick, I build the drawing with more carefully observed line work. The corner gives thinner or thicker lines to work with, depending on how you hold it. The hard, thin lines drawn over the soft, wide strokes, provide a nice contract of marks all from one tool.
And by "carefully observed line work", I mean drawing as fast as possible while not looking at the paper.