During the story you will run into spiders that attack you. In order to make them flip belly up, you have to hold your flashlight on them for several seconds. You will know the flashlight is working by the black trail that appears on them as you hold the beam on them.
Don't forget to keep your battery charged with the generator you carry, and use your lighter as well to burn up the webs that tend to be all over the spider areas.
Very much like Alan Wake combat tactics :)