Explanation
This is a memorization trick to help you remember how to convert a magnetic compass heading to a true heading, accounting for magnetic declination (also called variation). When declination is East, the compass reading is the "least" (smaller) of the two numbers, so you add the declination to get the true heading. When declination is West, the compass reading is the "best" (larger) of the two, so you subtract the declination to get the true heading. For example, with 3° of east declination, a compass reading of 270° magnetic corresponds to 273° true.