The eye icon is available on graded modules in Incorrect tabs and is only useful when you have multiple graded answer modules within the question.
The tab will only trigger if the student's answers match all of the modules with an open eye.
The tab can still trigger regardless of what the student puts for the modules with a closed eye.
Suppose you have a multi-part question, and want to give feedback a specific wrong answer in part 1. Leave the eye open on the answer module for part 1 and enter the specific wrong answer that you want give feedback for. On all the other answer modules, click the eye to "close" it. Now, this tab will trigger when the student's answer matches what's in part 1, regardless of what they put for the other parts. Add a text area (and any other relevant modules) to explain what the student should do to fix their incorrect answer in part 1.