If your Army Faction is GENESTEALER CULTS, you start the battle with a number of Resurgence points, depending on the battle size, as shown below.
Each time a unit from your army is destroyed, if every model in that unit has this ability, you can spend the relevant number of Resurgence points shown below based on that unit’s Starting Strength.
If you do:
Cult Ambush is a type of Strategic Reserves. Units in Cult Ambush cannot be targeted with the Rapid Ingress Stratagem, but can be set up on the battlefield using a Cult Ambush marker as described below, or by following the rules for Strategic Reserves in a subsequent turn. Units in Cult Ambush are not automically destroyed at the end of the third battle round.
Use a circular 32mm diameter marker for Cult Ambush markers. If an enemy model (excluding AIRCRAFT) ends any kind of move within 8" of a Cult Ambush marker you placed, that Cult Ambush marker is removed from the battlefield. At the end of your opponent’s next Movement phase, for each of your Cult Ambush markers still on the battlefield, you can select one unit from your army that is in Cult Ambush to make an ingress move. For that ingress move, set that unit up with at least one model in that unit in base contact with that Cult Ambush marker and all other models placed wholly within 3" of that Cult Ambush marker (that Cult Ambush marker is then removed from the battlefield). Units in Cult Ambush can make an ingress move in the first battle round using this rule.