It would seem tossing one's drink back was a sign of respect back then very much as it is now. The only difference is that nowadays downing your drink before your drinking partner shows respect, while in ancient China, you deferred to your drinking partner by drinking after him.
卒爵，也就是今人所謂的「乾杯」。看來，乾杯表敬意古今一致，但不同的是，今人以「先乾為敬」，古人則是「後乾為敬」。