Sentence:  Proficient drinkers could spill out an ounce, or an ounce and a half, or two, from the neck of a Solo Soda bottle in the dark, measured to the very dram, refill it with corn whisky, turn it upside down with the thumb placed carefully over the bottle's lip— for a good mix, and luck— and drink it down, <WSD>flatfooted</WSD>. 
Meaning of the word marked with <WSD> tags:  Without pausing between swallows.