Let me preface this response by saying that I watched the video without sound so he may be discussing what I am about to say. Trapping seems to be much more effective if the defender constantly shifts or shuffles forward into the opponent, as opposed to staying in the same area. It's almost like collapsing the attack. Essentially, after the initial block or trap, a straightblast would replace the trapping because the attacker's punches would be snuffed out by the constant counter attacks. Like Danny T said, "Trapping is a byproduct of striking." Again, that may be what the video was saying...without sound I know that I am missing that entire purpose of the video. I was just going by the visual. If my response has no connection to what is being shown in the video, my apologies.