The following is pulled from pg 18 of Cry Havoc 3 Gaming Aid.
The wreckers and the expert sappers have learned to pool their talents in order to demolish the enemy's rear guard.
For every expert sapper orc present in an army, a maximum of six wrecker dwarves can acquire the "Tunneller" ability. The wreckers' cost in A.P. is then increased by 1 A.P. each.
The wrecker dwarf tunnellers are not deployed with the rest of their army. They are set aside next to the battlefield.
At the end of each round 1d6 is rolled for each friendly expert sapper orc who is still on the battlefield. If one of the results is a "4" or more, then all of the army's wrecker dwarves can be deployed on the battlefield in the same way as for the "Reinforcement" ability.
If all expert sapper orcs are eliminated before the wreckers arrive, then the latter are considered to have been killed outright. A dreadful accident prevents them from joining the battle.