In the Pendulum, you simply bring your attention away from the porn to your own aroused body for five breaths. You can pause the porn or just close your eyes. As you breathe, scan through your body, noticing the places of most pleasure. During the Pendulum, most people continue to move and stimulate themselves, but some pay attention better with stillness. The point here is to place your attention on the delightful sensations within your body. After five breaths, return your attention to the porn.
Some people have difficulty turning away from the porn for even five breaths. If this is the case for you, notice your body for two breaths. Over time, the sensations of excitement and bliss in your body may seduce you into spending longer than five breaths while savoring.
Redirecting your attention from erotic visuals or fantasy to bodily feelings of sexual arousal takes practice. You are shifting from mental imagery toward direct felt experience in the body, and back again. This becomes easier after about 100 pendulums. Practice the pendulum as often as you like, provided it does not interfere with your sexual arousal.