Ambush is an odd skill. It depends solely on approaching and/or attacking your foe before he can react. There are any number of ways that this can be accomplished (being invisible, sneaking up on foe, etc..)
That requires a judgment call on the part of the GM to determine whether or not this was accomplished. Conversely, the GM can require an Ambush roll, treating it as a moving maneuver (5 ranks in Ambush giving a +25 bonus to the maneuver roll) with success meaning that the ambush was accomplished and the attack then proceeds before the foe can react
But if it was accomplished, then if the attack is successful and delivers a crit, the attacker is able to adjust the critical by up to a number equal to his number of ranks in the Ambush skill. (i.e. 10 ranks in ambush means you could adjust the critical from 1 to 10 points, either up or down the critical tables
For example, suppose you have 10 ranks of ambush, and you roll a "D" Crush critical and roll an 87. The crit adjustment (+10) adjusts this to 97. Ambush allows you make this anything from an 87 to a 107. You can select to make it a 100 (instant death) if you like by using only 3 points of your ambush.
I hope that helps.
