Per request, I uploaded some run defense out of the 4-2-5 Under to help compliment the other stuff I've shown from the formation.
4-2-5 Run Defense - YouTube
I can't stress enough how important gap integrity is. The bottom line is that when you user the free safety, you should only shoot a gap in this defense when you are 100 percent certain that you can make the TFL. I included a couple clips where it shows what getting overzealous will do.
The play art is very similar to that of the 4-3 Over Plus Run D that I posted a few days, and for good reason: IT WORKS!
Get in the lab, as this is one of my longer setups....even though its a very simple setup. The best part is that you have the time to audible into this look without switching formations if the opponent quick auds down into a power run.
Get to practicing, and best of luck!