When one has consumed a particularly dodgy curry or food dish, the results of which are for one to have a severly inflammed sphincter (hence 'jap flag'). There maybe some discomfort when passing faeces. Jap flag is often associated with sibling terms "turtle head (turtling)", "Ring-Stinger" and "touching cloth"
"That was a dodgy curry last night, I think I'm jap flagging"
"I am experiencing a case of Japanese flag"
"Excuse me your majesty, I must depart immediately, I'm touching cloth and being partiotic I hope I dont wave the jap flag"
"That ring stinger has left me with a severe case of Japanese flag"