"Trigger NOTAM" - one that tells you of a pre published change being activated.
AIP amendments in hard copy are actually pushed out 2 months in advance. So, if you know a change is happening, but for get when, the Trigger NOTAM brings it to your attention.
Having said all that - there is a school of thought that says they're actually pointless, as the changes wil normally be well briefed to concerned people. I for one agree.