We were talking before about free sag (Weight of the bike only). The spring is compressed slightly (compared to when the shock is off the bike). When mounted, The weight of the bike moves the shock piston into the shock body. Letís say it uses up 15% of the max travel. This is due to the weight of the bike. Free sag doesnít really matter and you donít adjust it. We care only about rider sag. When you sit on the bike (in full gear, feet off the ground), you want 30% of the travel to be used up. And so when you get on the bike, spring is compressed more. Letís say 50% of travel is now used up. Adjust (add) preload to get back to your 30% of travel point. Again, when adjusting preload, spring compression does not change. That only changes when the load changes (eg when you get on). By winding the preload ring towards the shock body you are simply driving the piston out of the shock. This makes it longer. Rear of bike rises. Ride height increases.
So once you have set your rider sag to 30% of shock travel, you are good. Now when the pillion gets on, load on the spring increases, it compresses, piston moves into shock body more, and more shock travel is used - lets say to 50% of max. To correct this , you add preload. In reality, when my wife gets on the back and we go to the pub, I donít mess about changing preload. If I were doing a track day with her on the back I would. So yes you are correct- adding preload with a pillion on board does not change spring compression - it just moves the piston out of the shock body and increases ride height. Just like when you add preload to the forks - you donít compress the spring, you just push the sliders out of the stanchion more.