In all honesty, both of the plays you hear are more than likely PI. What makes it hard for fans to understand is the difference in crews and how they choose to enforce it. As some will allow more than others.
Another reason that it isnt called more often is the volume of passing as compared to a few years ago. With more passing, more clock stoppage, and the NFL has a time in which they want games complete. That is a large reason why there probably won't be a change to OT rules as well.
But to try and answer your question, and I am certainly not great with words on how to explain something, its much easier to demonstrate, but with a crew who would be in the middle, as in not flag happy nor letting them get away with A LOT.
With both going back and forth, have to look at it as impeding the ability to make the catch. So holding and not letting go in a timely manner, striking before the ball arrives, and not giving the receiver the ability to get his hands up.
On most pass plays that take time to develop, there is holding, illegal contact, or PI. I have more of a problem with the blatant pick plays. As the rules have changed so much at the detriment of the defense. When that is allowed then its almost impossible