brain damage and caused the heart to stop. reporter: dr. martin tobin, an expert on breathing and lung function, testifying he watched videos of floyd s final moments hundreds of times, explaining what he saw when the officers pinned floyd down on the asphalt, handcuffed. it s like the left side is in a vice. it was almost to the effect as if a surgeon had gone in and removed the lung. reporter: dr. tobin testifying the officer s actions interfered with central features of how we breathe. to understand the knee on the neck, you need to examine your own necks, all of you here in the jury, like i am doing now. reporter: the jury captivated and most did touch their necks as the doctor spoke. he walked them through the video, frame by frame. pointing to chauvin s body position in this image. what you re seeing is that the toe of his boot is no longer touching the ground. this means that all of his body weight is being directed down at