Excuse my ignorance but...

Think of doing a rapid succession of more-or-less Wing Chun style straight punches to an opponent's face as you move forward. The idea is that they can't block them all and as you're moving forward while they're moving backward you have an advantage. These are not power punches, they're off-balancing punches to set up your HKE or other finish.

Let me note that this is based on my 6 months of JKD experience and perhaps someone else will correct me.
I think that's a pretty good way of describing it. My instructor used a good analogy: "If the right cross is your shotgun, the straight blast is like a machine gun." It's used to dominate the centerline and as a way to open the opponent up for bigger finishing shots. I'm sure it hurts to be nailed in the face (or chest or any other centerline target) also.

Actually, my instructor often hits me in the head (wearing headgear) with the straightblast when sparring. It's very disorienting and pretty much he has to end the engagement so I can try something else. I can only imagine that it would be more disruptive if I weren't wearing protective gear.

Bryan :)