Agressive slide protection pretty much spreads your blockers outward. It really opens up your gaps for the defense to blitz through but can slow down the pressure off the edge. I never use this when running but mix it in when Im in shotgun and dont read blitz. As for the wr screen problem, that could be a couple things. Either you have a slower/less agile reciever or you could be hitting speed burst too soon. Try catching the ball, finding the best whole, then running up field with speed burst.