A World Aflame; Overwatch, how to play?

[dice shaker]

Good afternoon,

 the rules "A World Aflame" contain the rule: Overwatch. But I don´t know how to use it. The problem for me is my moment to open fire. The rule say I choose the moment to open fire. Does this mean, that I stop the opposite player, pushing its tank or troop through my field of fire, or did I have to wait, till my oponent finish his move of that tank or member of his unit.
Thanks for reading, glad if you can help.

Yours, Sebastian

[has.been]

Example: Enemy vehicle crosses over at a cross road. It starts AND finishes its move out of sight of your troops. If you have put your troops on overwatch it means you have told them to cover the crossing IN CASE the enemy tries to cross & therefore you can blaze away as soon as they come in view, or before they disappear. If your troops were not on overwatch they would have been caught napping & not shot at the enemy. I hope this helps.

[dice shaker]

Good evening,

 thank you for the answear. On overwatch, I have to please the opponent to push his tank over the table, till I decide to open fire. If successful, I decide the position of the broken tank (roadblock).
 
   Yours,  Sebastian

[dice shaker]

Good evening,

 in melee you roll a D6 for each figure. Apply the following modifiers: ---. (See p. 50).
But there don´t stand, which of the two fighting figures win, just that the loser is a casualtie.
I believe the higher result of the D6 and the modifiers is the winner, please correct me, if I´m wrong.

 Yours,   Sebastian