There's straight blast the technique, and straight blast the concept.
The concept is that you have a temporally cyclical (ie. repeating) technique on the centerline with intensity and forward pressure. This is used as an entry to close the distance in the fight while suppressing your opponent, so you can use destructive close-quarter techniques. When you talk about straight blast the technique, you're usually talking about left and right vertical fists alternately into the guy's face, while you run straight in on them (aka "chup choi"). Picture your hands attached to the chain of a chainsaw, at two different points on the chain. Turn it on, holding it at face height, and run at the guy. That's roughly what it is.
In the JKD world it is traditionally done after a bit of dancing around in the open range. You might do a shin kick, Thai kick, or eyepoke, or achieve a Filipino destruction first, and as the guy starts to flinch you gear into the straightblast. The straightblast evokes further flinching (perhaps the guy covers up, straight-arms you (pushes you away) or even turns around) and you run him over, and finish him with either a barrage or headbutts, knees, and elbows, or something else. That's a simplification, but hey. This formula is called "RAT" by Paul Vunak.
There are always folks who will insist that the "straight blast" cannot be reduced to "a mere technique". So there's the concept of it as well, as I said.
An increasingly popular tactic in MMA, used by Thornton's Straight Blast Gym, is what has come to be known as the "Belfort Blast" or "Boxing Blast". It is basically a sequence of right and left crosses on the centerline. It seems to work very well in the ring, but of course folks will debate whether, in a "real fight" you should be using closed-fist strikes to the head at all.
