For me, it appears, at 50 on tackles the harder hits occur as a receiver is catching a pass and result in more passes being knocked incomplete...Seems that moving the tackle slider in either direction reduces the hard hits; this may also have something to do with how penalty sliders are adjusted, so I am still searching for the connections between sliders, penalty sliders and events on the field.
Maybe raising Defensive pass interference prevents the defender from attempting the tackle as the ball arrives, so fewer hard hits and balls knocked out?