What literary device does the author use to describe Johnny's mugging?

As already mentioned, the main literary device used is that of flashback,  when the narrator leaves the present moment to relate something that happened in the past. We can also say that Ponyboy, when narrating the story of Johnny's beating, uses a careful build-up technique, revealing what happened in stages. For instance, the first clue that something bad happened to Johnny is when Ponyboy finds his jacket which is described as having a stain 'the colour of rust' on it. The stain of course is blood, but Ponyboy chooses to refer to it obliquely. The badly-beaten Johnny first appears as a 'dark motionless hump' rather than as a person; this is an example of a de-humanizing image. In this way Ponyboy builds up slowly to the terrible revelation of the savage beating Johnny received at the hands of the Socs. The actual beating is never shown at all: we only see the grim results of it in the form of Johnny's battered body and bloodstained jacket.  It is obviously a very painful memory for Ponyboy, which is no doubt why he relates it in a somewhat roundabout manner to the sympathetically-listening Cherry.

As already mentioned in another answer, the story of this incident also uses the technique of foreshadowing, which is to say it hints at an event later in the book. This is when Johnny declares that 'he'd kill the next person who jumped him'. He ends up doing just that.