actually it really depends. sometimes you can use it to express your appreciation of sb's kind behavior. e.g when somebody helps you pick up your books on the ground you may use "emkoi". and sometimes when you want to ask somebody to do sth for you, you can also use this word. e.g when you want to ask sb. to let you through you can tap him/her on the back and say "emkoi".

Emkoi : Permisi .