So, the AMI/National Enquirer arrangement: catch and kill

Pecker, friend of Trump, would buy exclusive rights to the stories of his lust interests, housekeeper, and doorman... and then not run the stories.

Now under ordinary circumstances that seems like a perfectly legal thing to do, and probably is, BUT

Trump was running for the presidency, the timing of the contracts was such as to keep the stories from damaging his campaign, and thus they amounted to in-kind contributions in violation of Federal campain finance laws.