Q:

Why was this Denmark officer spared by Prussian soldiers?

In this movie 1864, a Denmark officer puts on a hood and yells "Spare me" when Prussian soldiers are about to kill him and he is spared.

Please fast forward to 8:30 to see this clip.
I thought putting up your hands was the way to surrender on a battlefield, or it was not the way back in 1864ish? 

A:

Dropping your weapon, putting hands up or prostrating oneself is a well known sign of surrender, especially among professional armies coming from the same (more or less) cultural background.
In this case, the soldier is in total panic and he does everything he can to look non-threatening. Hiding his face does help then, also it makes the enemies that terrified him to disappear. Eventually, you might say, that similar to the hood placed on the head of a convict, he doesn't want to see his killer.

