If true, how does this new rule reduce plays per game or injury risk?
A lot of hurry up teams just hurry up to the line to prevent substitutions. But the ball isn't snapped any quicker really. The QB might spend 10 or 15 seconds evaluating the defense and calling an audible. Meanwhile the clock runs down, and the defense can't get any substitutions made. We do this a lot. I don't think the rule would reduce the number of plays. And I'm not sure how it would reduce injuries.
It would probably only have a direct effect on the number of plays for hyper-tempo teams like Oregon. It also might generally slow down other teams because if the defense automatically has a chance to sub the offense might not push the pace quite so much.
I think the theory about injuries is players are more likely to get injured when they are tired. Thus, offenses going fast and now allowing the defense to sub creates more opportunities for injuries. They're saying allowing tired players to be subbed out should lessen the chance for injuries.