First and foremost you need to create the question that you plan to target back to and add it to the survey. Next, create and add the follow up question to the survey.
Once both questions have been added to the survey, identify your follow up question in the list of survey questions and click on the bullseye icon (the same one used in other targeting functionalities). A targeting pop up will appear that allows you to target previously selected answer choices and other actions based on participation. The bullseye button will turn purple once targeting has been added and the hover text will state that targeting has been enabled. You can review your in survey targeting in the survey review page.