Trolls can regenerate damage if they are not too badly hurt. Work this out as follows.

When Trolls are attacked calculate the number of wounds the unit suffers as normal. Once both sides involved in the combat have made all their attacks the Trolls may try to regenerate. Roll a D6 for each wound suffered during the combat. If you roll a 4 or more that wound has regenerated. Any regenerated wounds are reinstated, and models removed as casualties are replaced if enough wounds are recovered.

The results of combat are worked out after the Trolls have regenerated, and the number of wounds inflicted on them does not include any that are regenerated.

For example, three Trolls are fighting five Empire Knights. The Knights strike first and inflict 5 wounds, enough to kill one Troll and cause 2 further wounds. The remaining 2 Trolls inflict 3 wounds on the Knights. The Trolls now test to regenerate and successfully regenerate 3 wounds. The 3 wounds are reinstated, the slain Troll is replaced, and the 2 wounds suffered are noted down. In this example the Knights have scored only 2 wounds in the end while the Trolls have inflicted 3. Assuming no other combat bonuses apply the Trolls have won thanks to their regenerative abilities.