That's not Nerf jargon, it's gun jargon. Fanning a shot is when you hold down the trigger on a revolver and move your hand back over the hammer to rapidly (but less accurately) fire the revolver. Demonstrated here:
It sounds like what the reviewer is saying is that you can hold the trigger down on a strong arm and pull the slide back repeatedly to fire several shots. Since the slide is spring loaded and returns to it's resting position on it's own, you might be able to use a technique similar to fanning a revolver to rapidly fire a strongarm. Seems like an awkward way to fire the strongarm to me.
So basically what Nerf calls "slam fire". It's not really too awkward, but it is inaccurate. Plus I get the feeling I'll break the blaster, slam firing it.