A better description of what they do is to make a list of the features that car Modern has and car Old doesn't. Say the airbag. Or anti-lock breaks. Then they l

A better description of what they do is to make a list of the features that car Modern has and car Old doesn't. Say the airbag. Or anti-lock breaks. Then they look at how much it costs car Modern to add the airbag, using manufacturer's cost of producing that feature. Or how much it costs to have anti-lock breaks. Then they subtract that from the price of car Modern to try to get an apple to apple baseline of how much it would take to produce car Modern without the feature. Note that this process is not a hedonic adjustment, it's a production cost adjustment, which is what is used to deflate the price of cars.