overwatch is a play on defensive fire. during your movement or attack phase, you can declare that you are placing a unit in over watch which counts as the units action for that phase. What does this mean? When the unit is put into overwatch, the turrent or facing of the unit determines what hex row is in overwatch. Overwatch only applies to the row of hexes that the turrent is facing (both fixed guns and turreted) as far as the range of that particular unit. If an enemy unit moves through that hex row, the unit in overwatch can perform a defensive fire attack if it has line of sight to the enemy unit. For example, if you have a german 88 flak gun in overwatch and an enemy tank moves through the hex row in overwatch, the 88 can defensive fire with appropriate ranged AT value. Also, in the new rules, defensive fire can damage or destroy instead of only disrupting. Be careful with overwatch though, units in overwatch can not move until they are taken out of overwatch and they can only attack units that move through the hex row in overwatch. If you move first and place a unit in overwatch and your opponent moves units so they can attack the unit in overwatch, you can not move are attack the units in your attack phase. You have to use that units action to take it out of overwatch. Hope this helps.