A secondary progression aspect may be triggered by an aspect to one of the progressed planets from a transiting planet that is the traditional dispositor of the progressed planet.
Example: at the time of the Lewinsky scandal, Bill Clinton's progressed Venus (in Scorpio) square progressed Sun. When transiting Mars (ruler of Scorpio, dispositor of progressed Venus) hit by square progressed Venus, the scandal broke.
There is a progressed birthday each year. In addition to the "regular" birthday (=transiting Sun conjunct natal Sun), there is also another birthday, when the transiting Sun is conjunct the progressed Sun. Look for conjunctions between the transiting planets and the natal or progressed planets at the time of the regular or progressed birthdate. These conjunctions will hold a special importance during the entire year.
Example: in one case I know, at the time of the progressed birthday, the transiting Moon was conjunct the natal 5th cusp. The person conceived a child really soon afterwards (actually a week).
Progressed stations are important, as they bring important changes in life. A direct station (reversing motion from retrograde to direct) may be experienced like a resurrection in the lifea area governed by the planet (mental activities - Mercury, love life - Venus, drive and energy - Mars). A retrograde station may lead to experiencing again the same life situations, only from a different, more subjective perspective.
Besides secondary progressions, well known and widely used, tertiary and minor progressions are useful also. They just refer to a shorter time frame. For instance, someone may have one or none direct secondary progressed Venus stations during the lifetime, but will have two tertiary progressed and four minor progressed Venus stations.