When you decide to go from drawing figures that are posed to drawing a moving scene it will be much harder because you will not have the time to study moving figures the same as you would when the subject is posed on one spot for more than five minutes.
When drawing a posed object you must quickly map out the shapes and form of the figure and the same technique is required for drawing moving objects. It is probably more important just to capture shapes the object is forming rather than trying to catch all the minor details. However doing this though will not be good enough to capture someone moving, even if you are very quick at sketching.
The best thing to do in this situation is look at at your subject and take a photo in your minds eye, record them for a moment and remember their movement. Drawing this way may be very hard for you at first but like all things with drawing, with practice you will get better at it. As you do it more you will find it easier to remember the movement of the figure from a quick look.
Sometimes the movement of the figure may be repetitive which can help because you will have the opportunity to take a second glimpse and really capture the essence of the objects movement.
If you want to improve your speed of sketching and memory of movement then it is recommended that you grab a sketchbook and pencil. Head to somewhere where you know you can sit down and watch people in a busy environment but they are moving slow enough for you to remember their movement. A good example is going to a market, people will stop in front of stalls giving you an opportunity to capture them while they are still.
Also people will be moving slow because markets are normally quite busy. You do not have to be concerned too much with completing a figure, try to capture a quick essence of the figure and just simply move on to the next part of your drawing. Let the sketches overlap each other and within time a scene will start to build which you will be able to work on.