How the trigger works:
When the trigger is activated (and yeah, you can use the group function to link a trigger to a specific group, then select "vehicle" "leader" "any" or "all"), it looks at each unit capable of activating the trigger. For that unit, it gets its direction and speed, and camcreates shells. The [2,4,6,8,10] is the number of seconds before impact for each shell. So the first shell is dropped from a height it will cover in two seconds, and above the spot where, two seconds later, the unit will be if it maintains its heading and speed (+ a cheap amount of randomness -- I can improve that too).
The getpos bomb select 0 in the activation field is a trick: when a shell explodes, its location goes to [0,0,0], so the variable assigned to it (in this case bomb) will also go to [0,0,0]. When the trigger activates, the variable bomb is assigned to a bunch of different objects in sequence, but the last one is (unless you change the order of the numbers [2,4,6,8,10]) the last shell in the salvo to explode. When it explodes, the position of bomb goes back to [0,0,0] and the trigger resets, allowing yet another barrage.
You can do variations on this to create flak, or do any number of simple ofp effects.