Defensive assist basically lets the computer take over the player you're controlling.
It helps people who aren't that great at angles an tackling.
SO basically next time you're in route to a ball carrier... Take your tumb off the joystick and hit A.
The CPU will move the player for you.
In pass coverage the CPU will swat the ball for you, they wont intercept. And they wont ever get the strip for you by using the defensive assist.
It really helps in the backfield when QBs do that cheesy girating crap that cheesers do online to try to shake you.