A lot of people can't grasp how the lower teeth are supposed to keep up with the upper teeth. For me it was hard to understand as well, however, I think I've gotten a somewhat okay understanding of how teeth maintain occlusion. For the record I'm not a dentist nor do I have any professional background in this field.

Teeth, given enough contact (and near contact when chewing) will always fall into a certain occlusion. 

The cusps will always meet like this. The mandibular buccal cusp will meet the maxillary buccal pit, it's only natural, if you were to roll a ball down a hill like this 

The ball, rolling due to the forces of gravity, would naturally end up where the image illustrates, teeth are the same way. There are forces from the bone pushing the teeth out, in addition to the forces from the jaw pushing the lower teeth onto the upper teeth. Not only that, within the gums/alveolar bone that houses the teeth, the teeth are given a little give, a little leeway, in other words they aren't totally fixed into the bone and shift ever so slightly. This is why biting down on braces can feel so weird, they artificially hold the teeth in a fixed position when the teeth are not at all fixed. So what would happen when the top tooth for example moves to the left?

Now given that teeth are not fixed, that forces are pushing the teeth out as well as pushing the lower teeth onto the upper teeth when you close your mouth, that your tongue is maintaining your top tooth's new position, that the mandibular buccal cusp will naturally meet with the maxillary buccal pit, the bottom tooth will shift to the left as well and meet with the top tooth's new position.