Submit(to give over or yield to the power or authority of another)therefore to God. Resist(to withstand, strive against, or oppose, to withstand the action or effect of,to make a stand or make efforts in opposition; act in opposition; offer resistance) the devil and he will flee from you.
The Lord gave me a nice visual picture of how this works. Picture in your mind a medieval fortress with multiple gates. You are that fortress. When you submit to God you open one of those gates to let the power of God in, His Spirit and His angels into the fort to guard the ramparts and gun down the enemy outside the gates. When you resist the devil you keep the gates closed to him and maximize your defensive posture. If you yield to the devil, he enters the fortress (through the gate you opened to him) and you have a battle inside the castle walls, something you absolutely do not want. Made sense to me :-)