A quick, decisive move.

I believe that the "fell" in question is the 'fierce' or 'lethal' fell, coming from the Old French fel. "Swoop", as we all know, means to make a sudden attack.

Thus, "In one fell swoop, I stepped through the door" would probably be inappropriate usage.

"In one fell swoop, I moved my queen to rook 4" is borderline.

If you are a serious user of the English language, you should probably save this for sentences like "In one fell swoop, we fell upon the Huns and tore them limb from limb"

So now you know.