The embusen rule is basically that a kata should "balance". Each step in one direction should be countered by a step in the opposite direction. If they did the design of the kata right, you should always end up exactly back at the point you started. The kiten. It's good for performance of kata because you can run through a load of them without really moving, and in a small space, with a group of people. The limit to steps in any direction in okinawan kata is usually 3, giving approximately a 3mx3m (3yard x 3yard) training space.
However. Imagine you're designing your kata, and the embusen doesn't quite fit the embusen rule meaning you end up not on the kiten. What do you do?
1. Ignore it.
2. Fill with some "basics".
3. Something else?