It works like this:

You defend against the attack from the poster above you and then you attack the poster below you.

Example:

Poster #1

Attack: *Throws rock at next poster*

Poster #2

Defend: *Holds up garbage can lid to deflect the rock*

Attack: *Throws flaming wad of paper at next poster*

I'll start:

Attack: *Throws banana flambe at next poster* :