At the end of the series premiere, Nick believed that he had killed LaCroix. However, in the first episode of Season Two, LaCroix returns. He is determined to force Nick to move on, to which end he has carefully contrived to plant false evidence suggesting that Nick is a murderer.
Not realizing that LaCroix is back, Nick does not guess the truth until he is confronted by his master. The two fight, with the older vampire sure that he will prevail as he has always done, and the younger one desperate to prove his independence from his master's control.
When LaCroix seems to be winning, Nick grabs two long broken boards, on which the paint is peeling off. Holding one board in either hand, Nick positions them so that they cross roughly at right angles. The lengths of the sides are not quite equal. Nevertheless, the improvised cross successfully wards LaCroix off: he reels back, and then flies up out of reach.
Clearly, there is a matter of intent at work here: Nick wishes the crossed boards to symbolize the religious object, and it works. Indeed, it not only affects LaCroix, but also Nick. It is only with effort that he is able to maintain the cross shape, since—like any other religious artifact—it burns his hands as he holds it.