Imagine you are describing the object with your hands as you talk to someone - those hand gestures are very like those you make when gesture drawing. The marks are quick and deliberate. You look at the subject and try to sum it up with a few marks, as you might describe it in a few words. Because you don't have much time, each word - each mark - in a gesture drawing must say something significant about the subject.
When creating a gesture drawing, according to Kimon Nicolaides in 'The Natural Way to Draw', "you should draw, not what the thing looks like, but what it is doing. You need to 'sense' the thing that you are drawing. Is it fluid and soft, or spiky and hard? Is it coiled like a spring, or off-center and assymetric, or is it solid and balanced?
By nature gesture drawing tends to be done rapidly. Loose, often circular marks capture the flow of forms. Look at the whole object and notice points of tension, direction of weight or pressure, spaces, protrusions into space.