A unit on overwatch can also react to the reaction of an enemy unit on overwatch. In fact any number of units can react to the reaction of an enemy unit on overwatch.
And then other units on overwatch can react to that reaction. And so on and so forth. This is known as snowballing.
Reacting units do not carry out their reaction as it is declared. Just keep flipping markers and declaring reactions until both players have finished.
Only after the snowball has stopped is it time to determine what order reactions are taken in and to determine the actual outcome of the initial action that started the snowball and the reactions taken.
You do not have to remember the reaction declared for a unit. When it is a unit's turn to determine the reaction, you can change your mind as to exactly what reaction that unit takes.
Whichever unit that triggers a reaction or snowball of reactions is designated the instigator. That unit's action is only resolved once all reactions resulting from their action have been resolved, unless they have their hand forced.
Forcing the Instigator's Hand
A unit reacting directly to the instigator can force the instigator's hand.
Reacting directly means that the instigator is the target of the unit that is reacting. The player of the reacting unit informs their opponent that the instigating unit has their hand forced.
The instigating unit must immediately resolve their action before the reacting unit resolves their reaction.
ORDER OF REACTIONS
The order that units carry out a reaction is determined by the RE stat. Highest RE reacts first, lowest RE last.
Units With the Same RE
Sometimes both players will have units with the same RE that are reacting.
Both players delegate one of the units with the same RE to carry out their reaction. Both players then compete with a D6. Highest roll reacts before lowest roll. Keep rolling if both players get the same result.
The winner can choose to have their unit react after the loser's unit reacts. This is called hesitating.
Once those reactions are resolved, move on to the next units with the same RE.
When determining the outcome of a snowball, an overwatch freeze is in place. This means that other units still in overwatch that did not participate in the initialisation of a snowball cannot react and jump into the snowball.
You'll know which units are frozen, because their markers will still be face-down.
What reactions can a unit on overwatch take?
PROVIDE COVERING FIRE
Covering fire is the use of ranged weapons in order to pin or suppress enemy units so they cannot act that turn, whether with a reaction from overwatch, or an action from an action assignment.
Thrown weapons cannot be used to provide covering fire.
Covering fire is of no real use on enemy units that have already had an action in that turn. Providing covering fire burns the unit's action that provides the covering fire.
Covering fire is handled like normal shooting but with some very important differences.
When an enemy unit moves into base-to-base contact with an allied unit, a unit on overwatch that is within a distance equal to their SP or less of the allied unit, may intercept the attacking unit, as long as there is no obstruction (such as high cover) and as long as the two allied units have a line-of-sight or line-of- communication to each other.
A unit on overwatch can react to an enemy unit moving into base-to-base contact with them by cutting them off. This is making a ranged attack on the enemy unit, at any point in the enemy unit's move. Point blank range is usually the best option.
If a unit on overwatch cuts off an enemy unit, they may not retaliate in close combat that turn.