When the turbo starts creating boost some boost pressure is sent through the compressor discharge nipple and into a pipe.
This boost pressure will then travel down the pipe until it gets to your boost controller.
The bleed valve will bleed some of this boost pressure off so the remaining air flows down the pipe and into your actuator, depending on how much air had been bled off is how much pressure will now be sensed by the actuator.
A dawes device instead of bleeding the air off, the air now has to overcome a spring which you set by turning the device. The spring pressure will be reached at a certain boost pressure and it will allow the boost pressure to travel down and onto the actuator.
Hope this helps