I've watched this play 50 times and I can't wrap my head around it.  

So first off, ND needs to keep us from converting 3rd and long so they decide to bring pressure.  Not the worst idea in the world, but if you're gonna do that against this team you need to make sure you've got something to prevent a pass.  So with that in mind you go for 1 on 1 man coverage of the best WR in the world with no safety help?  I'm no football strategist, but that seems like the worst decision you could make. 

Then secondly, by all accounts Gray is a decent CB, if you're going to make the decision to put him 1 on 1 vs Smith you hope your guy can cover.  But Smith runs a simple route and gets 5-7 yards of separation.  I mean he just pulls away from Gray like it's nothing.  Gray doesn't seem to get tripped up or anything, he just gets absolutely smoked.  This is where I really can't understand things.  Is Smith just that much faster?  Is there something in the route I'm not seeing?  Did Gray do something wrong?  In this setting I just don't see how you let Smith get so open in such an obvious situation.  It's like going spread defense against the Eagles in 3rd and short.  Everyone knows what's coming and you don't defend it.  This just seems so blatantly obvious and ND coaches and players had to see this, I can't understand how they could fail so spectacularly.  Someone make it make sense (not that I'm complaining).