What does the coffee cup illustration have to do with the fullness of the Holy Spirit?
The coffee cup illustrates that at salvation a believer is filled to the brim with the Holy Spirit, just as a cup can be full of air. However, by choosing worldly allurements, temptations, and fleshly desires, a person can displace that fullness, filling the cup with material things instead.
Les teaches this analogy using a coffee cup full of liquid. To fill the cup with air, the liquid must be poured out. Pour out half, and half the cup is air; pour out three-fourths, and three-fourths is air. He connects this to the fullness of the Holy Spirit: the moment a person is saved, they are filled to the brim with the Holy Spirit. A newly saved person may be bubbling over, excited about what has happened. But believers remain in a world of allurements, temptations, and fleshly desires, so they can begin filling the cup—the pneuma, the Holy Spirit, likened to air—with material things. Soon the cup is full of the material instead.