The entire line is:
I'm not sure how I'm supposed to understand it. I think it means:
towards where will you swing the sword down right now (i.e. where are you aiming at? A person's arm? Their belly?)
What feels odd to me here is the use of
どこ. I can't visualize one swinging their sword down at somewhere, unless that somewhere is a part of something.
どこへ actually mean
何に or, perhaps,