First we deal with the case when there is no around advice present. Here each of the pieces of before advice are called in turn, with the same arguments that were given to the function, next the original definition is called with these arguments, and finally each of the pieces of after advice is called in reverse order with the same arguments (so that by default the most recently added piece of after advice is invoked last). The results returned by the function call are the values produced by the last piece of after advice to be called (if there is one), or by the original definition (if there is no after advice).
Note that none of these bits of code should destructively modify the arguments that they receive. Adding a piece of before advice thus provides a simple way of specifying some additional action to be performed before the original definition, and before any older bits of before advice. Adding a piece of after advice allows you to specify extra actions to be performed after the original definition, and after any older bits of after advice. The advice facility automatically links together these bits of advice with the original function definition.
LispWorks User Guide and Reference Manual - 21 Dec 2011