The Null Hardware override emulates an infinitely fast GPU by not sending any primitives to the GPU. This override is only available on systems using Intel® HD Graphics.
The Disable Draw Calls override emulates an infinitely fast graphics driver and GPU by not sending any primitives to the driver and, therefore, to the GPU as well. This can help you determine whether your application is CPU-bound or GPU-bound, depending on whether the performance significantly increases when using this override.
The Disable all option disables all override modes. Use this option to reset the rendering state of the game or graphics application back to the default state.
The Disable Draw Calls override discards the draw calls after they are submitted by the application. This has no influence on the "Draw Calls" metric, which measures all draw calls (even the ones discarded by the system) submitted by the application.