Sentence:  Parsival bounced upright and as the fellow reined close and chopped a swordstroke at his head the young knight, forgetting his own weapon in his rage, caught the <WSD>downstriking</WSD> arm with both hands and heaved the shocked man up out of his seat, across his own horse's neck, and spinning down the hill after his companion who was still rolling, quite slowly now, in the lush green field. 
Meaning of the word marked with <WSD> tags:  To move downward in a stiking motion.