Here is a detailed example (all three hexes are adjacents):
Hex A: 2 squads, one WT
Hex B: One Leader, 2 squads, one vehicle
Hex C: Three squads and one vehicle
By activating the leader hex in B and using his inherent leadership range of 1 (unless he is wounded), you can activate the two others adjacent hexes. Here is one possible serie of actions:
1) Activate hex B
2) Activate hex A & C
3) Fire with one squad in hex C
4) Fire with one squad in A and, at the same or another hex ,with the WT, resolving this as two different attacks
5) Fire with one squad in B
6) Assault Move the vehicle and move the leader and the squad that did not fired yet out of B to the same end location, using coordinated movement
7) Fire the just Assault Moved vehicle at the end of its movement
8) Move the vehicle in C to one end location and the two squads that did not fire to another end location (same end location for those two)
In each hex a maximum of one fire and one movement instances took place.Note the squad that did nothing in A is not considered activated and could be activated in a later impulse.
Sounds great? Imagine if there was units adjacent to B in six different hexes, some of them containing leaders that could, themselves, activate adjacent hexes in a sort of chain reaction...Sounds wonderfull?
That would be a very bad tactic, most of the time. You usually want to activate the least possible number of hexes, so that you can react to your opponent, opportunity fire and the like. But, in some occasion, you will use a stack to shaken the ennemy than another to enter into melee and eliminate the just shaken units before they can be moved away or reinforced.