To twofingermagic is to put two fingers, preferably the index and middle finger deep inside ones throat in order to produce a gagging reflex. One can only 'twofingermagic' voluntary, this is producing the gagging reflex without it is nessecary *yet*. You magically avoid being sick and having to vomit after drinking to much sauce.
Once nausiated to the point of vomiting, one can no longer perform twofingermagic, one can only vomit and be miserable about it.

the whale tail is the shape formed when a g-string rides up high over a womans trousers