Clearly outright liars who rape and then say, "but she clearly said yes" probably aren't going to get caught by these changes. But that's true of anyone who says clearly that x happened when actually y happened.
What I hope this change will do is reflect a societal change. Make men, and boys, think a bit more about how the authorities view a woman and her decision making regarding sex. It's clear that a lot of the problem with rape is that the perpetrators genuinely don't think they've done anything wrong, or they think they've only done "a little bit" wrong. If we can start to shift that thinking somewhat, we're starting to make progress.
The problem is that too often rapists are viewed as scary bogey men in the dark who jump out and attack random women. That's not true. Most rapists are "ordinary" men who know their victims. But who have somehow absorbed this idea that sex is their right and that short of a women screaming, scratching and drawing blood, it's not their obligation to be absolutely certain she wants it.
As someone upthread said, it's also about changing this perception that sex is something men want but that women just go along with. It should be clearly understood that sex is equally desirable for both and that means that both should be 100% engaged when it happens.