Secondary Progressions are based on the idea that the movement of one day’s worth of movement defines one year’s worth of developments.  In short, the planets/points are advanced one day’s worth of time for each year’s worth of time has passed until the time-period being examined.    There is an important specification here.  To get the angles and cusps/powerpoints to advance at the right rate, the Solar Arc in right ascension must be used.  Due to this being a slow, progressional system, the orbs for aspects here have to be quite small.  For sure, any aspects over 0° 15’ orb (for events) don’t need to be considered.  This system works 100% as well in the converse as in direct motion.