Thunder hammers release a terrific blast of energy when they strike an opponent. A thunder hammer counts as a power fist, but any model wounded by it and not killed may not attack again until the end of the next assault phase. Vehicles struck by a thunder hammer are 'crew shaken', in addition to any other results they suffer.
Errata — Chapter Approved: Codex: Space Marines Q&A (v3.0): The stun effect lasts until the end of the following Close Combat phase — victims may not strike back until then, which effectively means they strike last next turn, simultaneously with the Thunder Hammer bearer's next attack.