Without added weight, it's just a matter of relaxing your quadraceps and sinking naturally.
With added weight, I wouldn't advise this. You always want a slow and controlled drop all the way down to where your thigh and calf are touching. You can't do this with additional weight if your thighs are "turned off".
Then again, you could always ask a personal trainer at your local gym?
"I don't know if anyone is known with the art of "sitting on your couch" here, but in my eyes it is also to be a martial art.
It is the art of avoiding dangerous situations. It helps you to avoid a dangerous situation by not actually being there. So lets say there is a dangerous situation going on somewhere other than your couch. You are safely seated on your couch so you have in a nutshell "difused" the situation."