The real trick is to get the Guinness in as gently as possible and the spoon helps accomplish this by dispersing the liquid. Traditionally, you use the back of the spoon like the “black and tan spoon” shown below. However, I have found the best means to get the Guinness in there without disturbing anything is to have it trickle lightly down the inside of the glass. Using the front side of the spoon in contact with the glass seems to do the same, if not better.