Applying the brakes in hill descent control slows the crawl but remains engaged. IIRC as long as either X-mode is selected HDC will be engaged on a slope. If you give it some gas it will speed up and hold where you let off. If you step on the brakes it will slow down and continue at whatever lower speed is now set. Stepping on the gas or brake doesn't disable HDC unless you speed up to the point where X-mode disengages.