There are a number of things about Overwatch shots that are a little mysterious to me:
Aim Penalty
According to the description for Overwatch, there's an aim penalty:
Fire on the first enemy that moves within your line of sight, with a
penalty to aim.
What exactly is the penalty to aim? I've heard various people say -15% or even -30%, but I haven't seen any definitive confirmation.
Dash Defense?
In the previous XCOM game, XCOM: Enemy Unknown/Within, dashing provides a defense boost. Is this still true in XOMC 2, and if so, how much does a unit's defense increase?
Cover Defense?
If a unit was in cover before they move and trigger a reaction shot, does that unit still have the defense bonus that the cover provided, or do they lose it completely during the reaction shot?
Best Answer
The penalty is 30% (or x0.7) for a normal move and 40% (or x0.6) for a dash, it's multiplicative.
I.e. a 80% chance to hit turns to 56% for a normal move and 48% for a dash. Note, that when your normal hit chance is at least 70% it's actually better to take a shot at low cover than to use overwatch.
I don't know the exact interaction with the specialist GTS ability, cool under pressure, which grants +10 aim on overwatch shots. If it's applied before the overwatch penalty it's actually just a +6 or +7 aim.
Note, that concealed overwatch shots do NOT get any penalty.
Cover doesn't work on an overwatch shot, it would defeat the whole purpose of overwatch, if the target would still benefit from cover, as the cover and overwatch penalties together would cripple your hit chance. However you do not get a the 40% crit bonus from flanking the enemy.