When you are planning on passing another vessel from behind, there are several things you must do. Any vessel that is overtaking another must keep out of the way of the vessel being overtaken. This means that you can't expect them to move, slow down, or change their course. If you plan on passing them on THEIR PORT side, you must sound two short blasts. If they understand your signal and agree, they will sound two short blasts in response. The vessel being overtaken has a responsibility to maintain course and speed in order to minimize the potential for collision.