In college, the position of the ball is what counts. In the pros, the position of the player is what counts.
Thus, in college, when the ball breaks the plane, it's a touchback, REGARDLESS of where the player is or whether he has set foot in the end zone. The player could actually be standing in the endzone, and as long as he swats the ball back before it crosses the plane, it's not a touchback. In the pros, however, it's just the opposite. The player cannot touch the endzone, but can dive into it and swat the ball back to save it from touchback.
I've never understood why the rules are different, but they're pretty much the exact opposite.