"Reaction turn" has two uses. One you control, one you don't
In a sceanrio ,an AI formation with, for example 5 as its "reaction turn", is free to move as it sees fit when it reaches turn 5, UNLESS an objective is taken by the human player which triggers a counter-attack. That's the part you don't currently have full control over
Counter attack is based on the ratio of V-hexes NOT in the AI's hands
1] if 50% of the objectives are enemy held/neutral, there is a 5% chance EVERYONE (including react 99) units will charge
2] if between 75% and 90% of the objectives are enemy held/neutral (but not 100% ) - there is a 25% chance to counterattack with EVERYONE
3]if >=90% of the objectives are enemy held/neutral, the AI will attack with about 45-50% of the formations per turn
It's done this way TO BE unpredictable. There is no way for a player to know if tripping a V hex will set off a chain reaction and every game will be different in that regard. We have gone to GREAT lengths to ensure the game is NOT predictable. It's also why we don't like explaining these things becasue now people know the odds they are facing whereas in the past it was all a big mystery ( which was the intention....)
However, This is why you had trouble holding unit in place or predicting when it would move even though you had it set to react on "turn 99".
What we are looking at is adding a special reaction turn number that will force formations set to that number to ingore counter attack orders in scenarios.
Don