[1] The most common occurrence of eight in the box in the NFL involves the strong safety walking down from his position (moving to within) 10-15 yards off the line of scrimmage before the ball is snapped.
[2] From this tightened position he can offer the aforementioned run support as well as jam WRs and TEs, blitz the QB, or provide flat coverage.
Due to the superior athleticism of NFL players, it is not uncommon for the box safety to even provide deep coverage after the snap, giving the QB a pre-snap Cover 1 read but effectively transitioning into Cover 2 or another shell post snap.
While this is not the eight in the box strategy, it gives the same results without showing what your defense is before the snap of the ball.
In a base defensive set, there are 3 levels of defenders; line, linebackers, and safeties.