When you ask about something important, such as "what kind of things did your mother do that made you feel bad or unloved," he can't answer. Or "what is your interpretation of my issues with our marriage?" He doesn't know. He can't remember. He can't even give one example. Well, I should say, he won't.
From an abuser's perspective...
It's great to claim you have a bad memory because you can change the rules constantly. You can say things and claim you forgot you said them. You can make promises and claim you forgot you did that too. When your victim calls you out on something you said before, you can accuse her of trying to manipulate you. You never would have said something like that! She must have misunderstood or taken it out of context. If you claim you can't remember what you've said or done, it can't be held against you.
You can also let your victim know that her feelings, needs, and requests are completely unimportant to you without actually telling her that directly. You don't say that, of course, but she sees it because you can't seem to remember what she's communicated to you. What you do remember is everything she's ever done wrong. You can let her know that her flaws and mistakes are what you think are the most important.
Living with someone like this will make you think you are losing your mind. That's the abuser's intention, to make you question your own sanity and then hate yourself. I've found that the best way to counteract his claims that he "forgot" is to calmly state what he said and then to not ask if he remembers it. It just gives him the opportunity to lie. Then, if he claims he didn't say something, or he accuses you of "taking it out of context," just simply tell him that he can't claim to have a poor memory and then tell you that your recall is incorrect. You'll shut him up fast (or make him expose his malicious intent) if you insist that he choose which scenario he's sticking with.
"Either you have a "bad memory" and can't remember; therefore you can't claim that I'm getting it wrong, or you do remember what you said, and so your claim about having a bad memory is a lie."