The contraption of straps you are referring to is called a "shooting harness". It supports the back and is pretty comfy to use when shooting in the sitting position and provides a much steadier aide than a sling.
Fun pre PRS and NRL story - About 15 years ago Ryan Kerr was able to get into what I call a "gumby" position. This is almost laying down forward while sitting then being fully supported with both elbows on the ground. He'd always win the sitting stages. So the Phoenix club decided to run a rope across the line in front of us for the sitting position stage with the new rule that the muzzle had to be above the rope. BTW I wasn't involved in this decision at all although I used to tease Ryan some about it.
Then one time in a sitting stage I decided to game it and use my harness and bum bag like I do in FT, LOL! Nope they saw me coming up to the line with the contraption and told me no it wasn't allowed so I didn't use it although it was still attached on me so I threw the bum bag(also used in FT) onto the ground to sit on when it was go time and they were kicking the bum bag out from under me, LOL! I'm looking back at them yelling at me thinking what the heck are you doing. I got the picture that it wasn't allowed either. Nice try on my part though.