You can't say "use this feature for testing".
You only can edit the field values it uses for testing.
Let's look at an example: Just return the count of features in a related table.
With the default $feature:
If I change the value of the field that is used for the relationship (it was 724):
Think about it this way: When you start the expression editor, it loads the values of the first feature and uses them to test and validate the expression (at least that's what I think it does). At this point, it doesn't have an actual connection to the feature anymore, it just uses the values. So to test the expression on different features, you can't change the feature it uses, you can only change the values.
Have a great day!
Johannes