Same principle as deloading. You need to recharge the batteries and allow for adaptation. A block of stimulative workouts will run experienced lifters into the ground. This is more a dual factor principle. For new or inexperienced lifters basic supercompensation/single factor programs will work well enough. I think HST refers to this as strategic deconditioning (I'm not very knowledgable on HST specifics) but many programs refer to this as deloading where the loading phase is the block that stimulates adaptation, the deloading phase allows recovery to take place.
You got it right, madcow. HST calls is strategic deconditioning. Really, the word strategic isn't needed. I think Bryan was just trying to sound intelligent. Seriously though, rather than lifting, you take time off entirely (9-14 days) before lifting again. With deloading, however, you continue to lift, just with a huge decrease in volume.
That's what I've gathered from the 5x5 routine, anyhow.