Comes from the party term, 'to tap a keg'.. where one inserts a tap into a keg and drinks what comes out. But the user is instead tapping an ass..
(1) fuck a female in the ass
(2) ejaculate in her anal cavity
(3) insert a beer tap into her anus
(4) pump the beer tap
(5) place the nozzle over her open mouth
(6) fill her mouth with the cum/ass-juice concoction
(7) hold her nose and mouth shut until she swallows