The only Hellboy comic I've read is "The Corpse" (25 cents!), but I think it was adequately explained in the movie - Hellboy gets out a piece of odd-looking tech and says he's looking for "a talker". Sure, it seems kind of sudden, but the movie has already established over sixty years of...
It was a bit of a jarring edit, but I saw it as the concussion knocking the good guys out, then Elsa and Rsputin comes in and find Myers regaining consciousness.