The Portal is a break in spacetime which the Emewafwawa used to his advantage. The MRE (Music Rubato Energy) is something which resonates through Minas Music and is what lured the Emewafwawa to Earth because our world had abnormal amounts of this energy. He used this energy to cave into the Distortion World, in which he created his army of evil instruments. He feeds off MVE but that is not always enough, they sometimes need raw flesh and that is why the Emewafwawa Contraption Trap was created.
The portal is always open but it is only big enough for Emewafwawa to pass through it and its to small for any kind of instruments to pass through it. This is because Giratina doesn't like uninvited guests in the Distortion World, but he doesn't want to kill anyone.
The portal is situated inside a Double Bass case which is actually a generator which managed to detach it to mak it portable in the Double Bass case and it is where the Emewafwawa channels most of the MVE, as that is the only thing stopping the portal from being fully shut by Giratina.