The cyclinders in which the plugs are situated are in different phases of the ignition cycle. One will on the compression pahse, ready to ignite with a high breakdown voltage (about 10kv) due to the (approximate) 10 atmospheres pressure and the other will be in an exhaust phase with a relatively low pressure and thus a low breakdown voltage(about 1kv). The bodies of the plug (ground electrode) are joined by the metallic engine block. The secondary of the ignition coil has 2 terminals, one from each end of its coil. One goes to the hot terminal of plug A, the other to the hot terminal of plug B. The current flows from coil through 'A' to ground to 'B' back to coil. NOTE: There is no distriutor in this arrangement. That is the whole point.