[CentOS] Understanding RPM trigger scripts?

I'm trying to get my head around RPM trigger scripts - but currently 
getting very confused ...

I have a custom RPM which has in its spec file a '%triggerin' script for 
the 'kernel' RPM - i.e. a script that I want to run when the kernel RPM 
is upgraded/installed

This works OK - once my custom RPM is installed, if the kernel is 
subsequently upgraded/installed, my trigger script is run

However, the same trigger script is also run when I upgrade my custom 
RPM - which I don't want - and so far, I can't find a reliable way of 
making sure it isn't run in this case. The more I search for an answer, 
the more I get confused :-)

I know this is a bit off-topic and obscure, but does anyone know how to 
make sure such a trigger script is only run when the RPM that triggers 
it is upgraded/installed - and not when the RPM that contains the 
trigger script is upgraded?

Thanks

James Pearson