While perusing drink recipes, you will often see the phrase "Float (liquor) on top". This refers to adding the liquor so that there is no turbulence and mixing of the drink - the new liquor will be a distinct seperate layer sitting on top of the other components of the cocktail.

The easiest way to float one liquor on top of another is to use a medium sized syringe. Obviously this may not be available or desirable, so the common method is to use a spoon, holding it in the glass and slowly trickling the ingredient over the back of the spoon. If no spoon is available, it can be done by slowly pouring down the side of a glass (or alternately, pour down a stir stick that is touching the side of the glass.) Layering is a difficult business.