Questions about bombs

I have a hard time understanding the text for the bomb.

In the case of Cluster bombs for example, does the bomb blow up if a PS3 Deathfire drops a bomb under some PS9 Soontir? Or does it only blow up if this Soontir then starts his activation then flies over it (similar to starting your activation on an asteroid)

The reference card was sort of hard to understand.

Thanks a lot.