Gen12 (aka Xe) will hit the market in 2020 via the Tiger Lake (and possibly Rocket Lake) processors as well as the Xe discrete video cards.
In the patches, DSB is generically described as a hardware capability that will be introduced in the Gen12 display controller. This engine will only be used for some specific scenarios for which it will deliver performance improvements, and after completion of its work, it will be disabled again.More details at Tom's Hardware.
Some additional (technical) documentation of the feature is available, but the benefits of the DSB are described as follows: “[It] helps to reduce loading time and CPU activity, thereby making the context switch faster.” In other words, it is new engine that offloads some work from the CPU and helps to improve context switching time.