There are certain things that turning high motor off can be seen easily. The best example is on the OLine. With high motor on the lineman often have the ability to pickup blocks that they should not be able to pick. They simply more to places they should not be able to and block an oncoming pass rusher. However, turn High Motor off and that decreases dramatically. That is just one example.
I understand that having a "high motor" is important in sports, but that does not mean every player consistently plays like it. According to EA, the trait is less about "taking a play off" and more about the ability to stay involved in a given play. In other words continuing to make the play even after they would otherwise be out. Regardless of its use, the problem in terms of the game is that if everyone, or nearly everyone, has the trait, then there is no reason for the trait to exist at all. So, I and many others, have suggested using the trait more sparingly to truly differentiate between players.
Having built rosters from the ground up and using and not using traits over several iterations of Madden, I think I have a pretty good idea of what the traits do in terms of game play. You can choose to believe it or not. I, and others here, have spent countless hours testing the traits in a variety of settings as well as in full games and seasons. I will not argue the point further.