The situation: first quarter, first-and-10 on the Seahawks 30-yard line, Cardinals leading 14-0. The Cardinals jumped to an early lead on a long touchdown pass and a Seahawks turnover.
With Clancy Pendergast running the show for the Cardinals defense, Arizona breaks in a 3-3-5. The Cardinals go small, with Dansby coming up to the line and Adrian Wilson acting as a linebacker. The Hawks bunch Burleson, Darrell Jackson, and D.J. Hackett on the right side with Branch alone out left and Shaun Alexander in the backfield. Based on the zone the Cardinals drop into, there are two linebackers, a corner, Adrian Wilson, and two safeties left responsible for the offensive right, the middle, and anything deep.
Burleson and Jackson play important roles in this route combination. Jackson maintains the depth of the linebackers; as long as he is crossing the middle of the field and threatening their zones, they cannot release deep. Burleson's route draws Wilson away from Hackett. If Burleson had run a simple flat route, Wilson may have retreated as soon as Jackson left his zone, putting him in position to disrupt any deep pass. With Burleson threatening (and Alexander still in the picture on the offensive right side), Wilson is effectively neutralized.
That leaves the combination of Aaron Francisco and Robert Griffith to cover the streaking Hackett. Griffith has to shade toward Branch down the left sideline, and Burleson's route requires a fraction of Francisco's attention. As Hackett angles in, he naturally splits both defenders for the catch.
While the deep ball to Hackett worked effectively against the zone in this case, we can see the importance of play design: had the Cardinals come out in man coverage, both Burleson and Jackson would have had separation due to the rub/pick caused by their cross. If Hasselbeck reads man instead of zone, he then has a drag over the middle against a linebacker or single coverage on Hackett or Burleson, depending on how Francisco reads the play.