when something doesnt make sense in 40k, try looking at it from a different angle. in this case, a crewmen of said tank.
but first, the thunder hammer-
1-it creates a huge noise and flash when it hits something. its designed to do that.
2-its being weilded by a mammoth of a man, made even more mammoth by the termy armour.
3-have you ever hit a sheet of metal with a rock? now hit it with a hammer the size of you
now all of this together, and then the crewmens heads arent that far away from the hit, and youre going to be dazed for a little while. and that little while is enough lull in shooting so that the thing cant fire.
i had that same sort of "wait a minute, what?" thing when i realized you could kill a land raider with a harliquins kiss. at f
irst glance, it just doesnt make sense.