Triggers are a way to remember a song or chord progression, independent of a particular key. It is a pathway or road map through a song. I've always had trouble remembering songs and was amazed at how most pro musicians can remember so many songs.

The trick is to remember the very common harmonic sequences, be able to identify a chord's function within a tonalty and apply roman numerals to these chords. So when starting at A, how to get to B and pathway to get you there. Like a maze or directions when travling.

Look for cycles, modulations an repeating sections.

Here is an example using a very simple standard song Blue Bossa by Kenny Durham.

The most common key is Cm

Cm7	Cm7	Fm7	Fm7
Dm7b5	G7	Cm7	Cm7
Ebm7	Ab7	Dbmaj7	Dbmaj7
Dm7b5	G7	Cm7	Cm7

This can be summarized as:

Cm: I	I	IV	IV
II	V	I	I
Db: II	V	I	I
Cm: II	V	I	I

And further summarized keyless as:

I IV II V I ↑m3 II V I ↑m2 II V Im

The key to this notation if found on every song page.

And in english if you where explaining it to a fellow musician that has a little of the principles of how chord progressions work would be:

It is a I IV II V I on minor then up a minor third and them II V I in a major key. Up 1/2 step and II V minor to the I of the key. That is the whole harmonic structure of Blue Bossa.