Sentence:  From above one could only occasionally catch a glimpse of life on the floor of this green sea : a neighbor 's gingham skirt flashing into sight for an instant on the path beneath her grape-arbor , or the movement of hands above a clothesline and the <WSD>flutter</WSD> of garments hung there , half-way down the block . 
Meaning of the word marked with <WSD> tags:  the act of moving back and forth